[Intel-gfx] [PATCH 1/6] locking/lockdep: restore cross-release checks
Daniel Vetter
daniel.vetter at ffwll.ch
Thu Oct 25 14:35:22 UTC 2018
This reverts the following commits:
commit 527187d28569e39c5d489d6306d3b79605cf85a6
Author: Ingo Molnar <mingo at kernel.org>
Date: Mon Jan 8 17:27:19 2018 +0100
locking/lockdep: Remove cross-release leftovers
commit dba04eb76df982703fefc021a4d278347b6176a9
Author: David Sterba <dsterba at suse.com>
Date: Mon Jan 8 16:27:31 2018 +0100
locking/Documentation: Remove stale crossrelease_fullstack parameter
commit e966eaeeb623f09975ef362c2866fae6f86844f9
Author: Ingo Molnar <mingo at kernel.org>
Date: Tue Dec 12 12:31:16 2017 +0100
locking/lockdep: Remove the cross-release locking checks
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
---
.../admin-guide/kernel-parameters.txt | 3 +
Documentation/locking/crossrelease.txt | 874 ++++++++++++++++++
include/linux/completion.h | 46 +
include/linux/irqflags.h | 4 +
include/linux/lockdep.h | 127 +++
include/linux/sched.h | 11 +
kernel/locking/lockdep.c | 652 ++++++++++++-
kernel/sched/completion.c | 5 +
lib/Kconfig.debug | 33 +
9 files changed, 1720 insertions(+), 35 deletions(-)
create mode 100644 Documentation/locking/crossrelease.txt
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 92eb1f42240d..6ab3ad188c01 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -724,6 +724,9 @@
It will be ignored when crashkernel=X,high is not used
or memory reserved is below 4G.
+ crossrelease_fullstack
+ [KNL] Allow to record full stack trace in cross-release
+
cryptomgr.notests
[KNL] Disable crypto self-tests
diff --git a/Documentation/locking/crossrelease.txt b/Documentation/locking/crossrelease.txt
new file mode 100644
index 000000000000..bdf1423d5f99
--- /dev/null
+++ b/Documentation/locking/crossrelease.txt
@@ -0,0 +1,874 @@
+Crossrelease
+============
+
+Started by Byungchul Park <byungchul.park at lge.com>
+
+Contents:
+
+ (*) Background
+
+ - What causes deadlock
+ - How lockdep works
+
+ (*) Limitation
+
+ - Limit lockdep
+ - Pros from the limitation
+ - Cons from the limitation
+ - Relax the limitation
+
+ (*) Crossrelease
+
+ - Introduce crossrelease
+ - Introduce commit
+
+ (*) Implementation
+
+ - Data structures
+ - How crossrelease works
+
+ (*) Optimizations
+
+ - Avoid duplication
+ - Lockless for hot paths
+
+ (*) APPENDIX A: What lockdep does to work aggresively
+
+ (*) APPENDIX B: How to avoid adding false dependencies
+
+
+==========
+Background
+==========
+
+What causes deadlock
+--------------------
+
+A deadlock occurs when a context is waiting for an event to happen,
+which is impossible because another (or the) context who can trigger the
+event is also waiting for another (or the) event to happen, which is
+also impossible due to the same reason.
+
+For example:
+
+ A context going to trigger event C is waiting for event A to happen.
+ A context going to trigger event A is waiting for event B to happen.
+ A context going to trigger event B is waiting for event C to happen.
+
+A deadlock occurs when these three wait operations run at the same time,
+because event C cannot be triggered if event A does not happen, which in
+turn cannot be triggered if event B does not happen, which in turn
+cannot be triggered if event C does not happen. After all, no event can
+be triggered since any of them never meets its condition to wake up.
+
+A dependency might exist between two waiters and a deadlock might happen
+due to an incorrect releationship between dependencies. Thus, we must
+define what a dependency is first. A dependency exists between them if:
+
+ 1. There are two waiters waiting for each event at a given time.
+ 2. The only way to wake up each waiter is to trigger its event.
+ 3. Whether one can be woken up depends on whether the other can.
+
+Each wait in the example creates its dependency like:
+
+ Event C depends on event A.
+ Event A depends on event B.
+ Event B depends on event C.
+
+ NOTE: Precisely speaking, a dependency is one between whether a
+ waiter for an event can be woken up and whether another waiter for
+ another event can be woken up. However from now on, we will describe
+ a dependency as if it's one between an event and another event for
+ simplicity.
+
+And they form circular dependencies like:
+
+ -> C -> A -> B -
+ / \
+ \ /
+ ----------------
+
+ where 'A -> B' means that event A depends on event B.
+
+Such circular dependencies lead to a deadlock since no waiter can meet
+its condition to wake up as described.
+
+CONCLUSION
+
+Circular dependencies cause a deadlock.
+
+
+How lockdep works
+-----------------
+
+Lockdep tries to detect a deadlock by checking dependencies created by
+lock operations, acquire and release. Waiting for a lock corresponds to
+waiting for an event, and releasing a lock corresponds to triggering an
+event in the previous section.
+
+In short, lockdep does:
+
+ 1. Detect a new dependency.
+ 2. Add the dependency into a global graph.
+ 3. Check if that makes dependencies circular.
+ 4. Report a deadlock or its possibility if so.
+
+For example, consider a graph built by lockdep that looks like:
+
+ A -> B -
+ \
+ -> E
+ /
+ C -> D -
+
+ where A, B,..., E are different lock classes.
+
+Lockdep will add a dependency into the graph on detection of a new
+dependency. For example, it will add a dependency 'E -> C' when a new
+dependency between lock E and lock C is detected. Then the graph will be:
+
+ A -> B -
+ \
+ -> E -
+ / \
+ -> C -> D - \
+ / /
+ \ /
+ ------------------
+
+ where A, B,..., E are different lock classes.
+
+This graph contains a subgraph which demonstrates circular dependencies:
+
+ -> E -
+ / \
+ -> C -> D - \
+ / /
+ \ /
+ ------------------
+
+ where C, D and E are different lock classes.
+
+This is the condition under which a deadlock might occur. Lockdep
+reports it on detection after adding a new dependency. This is the way
+how lockdep works.
+
+CONCLUSION
+
+Lockdep detects a deadlock or its possibility by checking if circular
+dependencies were created after adding each new dependency.
+
+
+==========
+Limitation
+==========
+
+Limit lockdep
+-------------
+
+Limiting lockdep to work on only typical locks e.g. spin locks and
+mutexes, which are released within the acquire context, the
+implementation becomes simple but its capacity for detection becomes
+limited. Let's check pros and cons in next section.
+
+
+Pros from the limitation
+------------------------
+
+Given the limitation, when acquiring a lock, locks in a held_locks
+cannot be released if the context cannot acquire it so has to wait to
+acquire it, which means all waiters for the locks in the held_locks are
+stuck. It's an exact case to create dependencies between each lock in
+the held_locks and the lock to acquire.
+
+For example:
+
+ CONTEXT X
+ ---------
+ acquire A
+ acquire B /* Add a dependency 'A -> B' */
+ release B
+ release A
+
+ where A and B are different lock classes.
+
+When acquiring lock A, the held_locks of CONTEXT X is empty thus no
+dependency is added. But when acquiring lock B, lockdep detects and adds
+a new dependency 'A -> B' between lock A in the held_locks and lock B.
+They can be simply added whenever acquiring each lock.
+
+And data required by lockdep exists in a local structure, held_locks
+embedded in task_struct. Forcing to access the data within the context,
+lockdep can avoid racy problems without explicit locks while handling
+the local data.
+
+Lastly, lockdep only needs to keep locks currently being held, to build
+a dependency graph. However, relaxing the limitation, it needs to keep
+even locks already released, because a decision whether they created
+dependencies might be long-deferred.
+
+To sum up, we can expect several advantages from the limitation:
+
+ 1. Lockdep can easily identify a dependency when acquiring a lock.
+ 2. Races are avoidable while accessing local locks in a held_locks.
+ 3. Lockdep only needs to keep locks currently being held.
+
+CONCLUSION
+
+Given the limitation, the implementation becomes simple and efficient.
+
+
+Cons from the limitation
+------------------------
+
+Given the limitation, lockdep is applicable only to typical locks. For
+example, page locks for page access or completions for synchronization
+cannot work with lockdep.
+
+Can we detect deadlocks below, under the limitation?
+
+Example 1:
+
+ CONTEXT X CONTEXT Y CONTEXT Z
+ --------- --------- ----------
+ mutex_lock A
+ lock_page B
+ lock_page B
+ mutex_lock A /* DEADLOCK */
+ unlock_page B held by X
+ unlock_page B
+ mutex_unlock A
+ mutex_unlock A
+
+ where A and B are different lock classes.
+
+No, we cannot.
+
+Example 2:
+
+ CONTEXT X CONTEXT Y
+ --------- ---------
+ mutex_lock A
+ mutex_lock A
+ wait_for_complete B /* DEADLOCK */
+ complete B
+ mutex_unlock A
+ mutex_unlock A
+
+ where A is a lock class and B is a completion variable.
+
+No, we cannot.
+
+CONCLUSION
+
+Given the limitation, lockdep cannot detect a deadlock or its
+possibility caused by page locks or completions.
+
+
+Relax the limitation
+--------------------
+
+Under the limitation, things to create dependencies are limited to
+typical locks. However, synchronization primitives like page locks and
+completions, which are allowed to be released in any context, also
+create dependencies and can cause a deadlock. So lockdep should track
+these locks to do a better job. We have to relax the limitation for
+these locks to work with lockdep.
+
+Detecting dependencies is very important for lockdep to work because
+adding a dependency means adding an opportunity to check whether it
+causes a deadlock. The more lockdep adds dependencies, the more it
+thoroughly works. Thus Lockdep has to do its best to detect and add as
+many true dependencies into a graph as possible.
+
+For example, considering only typical locks, lockdep builds a graph like:
+
+ A -> B -
+ \
+ -> E
+ /
+ C -> D -
+
+ where A, B,..., E are different lock classes.
+
+On the other hand, under the relaxation, additional dependencies might
+be created and added. Assuming additional 'FX -> C' and 'E -> GX' are
+added thanks to the relaxation, the graph will be:
+
+ A -> B -
+ \
+ -> E -> GX
+ /
+ FX -> C -> D -
+
+ where A, B,..., E, FX and GX are different lock classes, and a suffix
+ 'X' is added on non-typical locks.
+
+The latter graph gives us more chances to check circular dependencies
+than the former. However, it might suffer performance degradation since
+relaxing the limitation, with which design and implementation of lockdep
+can be efficient, might introduce inefficiency inevitably. So lockdep
+should provide two options, strong detection and efficient detection.
+
+Choosing efficient detection:
+
+ Lockdep works with only locks restricted to be released within the
+ acquire context. However, lockdep works efficiently.
+
+Choosing strong detection:
+
+ Lockdep works with all synchronization primitives. However, lockdep
+ suffers performance degradation.
+
+CONCLUSION
+
+Relaxing the limitation, lockdep can add additional dependencies giving
+additional opportunities to check circular dependencies.
+
+
+============
+Crossrelease
+============
+
+Introduce crossrelease
+----------------------
+
+In order to allow lockdep to handle additional dependencies by what
+might be released in any context, namely 'crosslock', we have to be able
+to identify those created by crosslocks. The proposed 'crossrelease'
+feature provoides a way to do that.
+
+Crossrelease feature has to do:
+
+ 1. Identify dependencies created by crosslocks.
+ 2. Add the dependencies into a dependency graph.
+
+That's all. Once a meaningful dependency is added into graph, then
+lockdep would work with the graph as it did. The most important thing
+crossrelease feature has to do is to correctly identify and add true
+dependencies into the global graph.
+
+A dependency e.g. 'A -> B' can be identified only in the A's release
+context because a decision required to identify the dependency can be
+made only in the release context. That is to decide whether A can be
+released so that a waiter for A can be woken up. It cannot be made in
+other than the A's release context.
+
+It's no matter for typical locks because each acquire context is same as
+its release context, thus lockdep can decide whether a lock can be
+released in the acquire context. However for crosslocks, lockdep cannot
+make the decision in the acquire context but has to wait until the
+release context is identified.
+
+Therefore, deadlocks by crosslocks cannot be detected just when it
+happens, because those cannot be identified until the crosslocks are
+released. However, deadlock possibilities can be detected and it's very
+worth. See 'APPENDIX A' section to check why.
+
+CONCLUSION
+
+Using crossrelease feature, lockdep can work with what might be released
+in any context, namely crosslock.
+
+
+Introduce commit
+----------------
+
+Since crossrelease defers the work adding true dependencies of
+crosslocks until they are actually released, crossrelease has to queue
+all acquisitions which might create dependencies with the crosslocks.
+Then it identifies dependencies using the queued data in batches at a
+proper time. We call it 'commit'.
+
+There are four types of dependencies:
+
+1. TT type: 'typical lock A -> typical lock B'
+
+ Just when acquiring B, lockdep can see it's in the A's release
+ context. So the dependency between A and B can be identified
+ immediately. Commit is unnecessary.
+
+2. TC type: 'typical lock A -> crosslock BX'
+
+ Just when acquiring BX, lockdep can see it's in the A's release
+ context. So the dependency between A and BX can be identified
+ immediately. Commit is unnecessary, too.
+
+3. CT type: 'crosslock AX -> typical lock B'
+
+ When acquiring B, lockdep cannot identify the dependency because
+ there's no way to know if it's in the AX's release context. It has
+ to wait until the decision can be made. Commit is necessary.
+
+4. CC type: 'crosslock AX -> crosslock BX'
+
+ When acquiring BX, lockdep cannot identify the dependency because
+ there's no way to know if it's in the AX's release context. It has
+ to wait until the decision can be made. Commit is necessary.
+ But, handling CC type is not implemented yet. It's a future work.
+
+Lockdep can work without commit for typical locks, but commit step is
+necessary once crosslocks are involved. Introducing commit, lockdep
+performs three steps. What lockdep does in each step is:
+
+1. Acquisition: For typical locks, lockdep does what it originally did
+ and queues the lock so that CT type dependencies can be checked using
+ it at the commit step. For crosslocks, it saves data which will be
+ used at the commit step and increases a reference count for it.
+
+2. Commit: No action is reauired for typical locks. For crosslocks,
+ lockdep adds CT type dependencies using the data saved at the
+ acquisition step.
+
+3. Release: No changes are required for typical locks. When a crosslock
+ is released, it decreases a reference count for it.
+
+CONCLUSION
+
+Crossrelease introduces commit step to handle dependencies of crosslocks
+in batches at a proper time.
+
+
+==============
+Implementation
+==============
+
+Data structures
+---------------
+
+Crossrelease introduces two main data structures.
+
+1. hist_lock
+
+ This is an array embedded in task_struct, for keeping lock history so
+ that dependencies can be added using them at the commit step. Since
+ it's local data, it can be accessed locklessly in the owner context.
+ The array is filled at the acquisition step and consumed at the
+ commit step. And it's managed in circular manner.
+
+2. cross_lock
+
+ One per lockdep_map exists. This is for keeping data of crosslocks
+ and used at the commit step.
+
+
+How crossrelease works
+----------------------
+
+It's the key of how crossrelease works, to defer necessary works to an
+appropriate point in time and perform in at once at the commit step.
+Let's take a look with examples step by step, starting from how lockdep
+works without crossrelease for typical locks.
+
+ acquire A /* Push A onto held_locks */
+ acquire B /* Push B onto held_locks and add 'A -> B' */
+ acquire C /* Push C onto held_locks and add 'B -> C' */
+ release C /* Pop C from held_locks */
+ release B /* Pop B from held_locks */
+ release A /* Pop A from held_locks */
+
+ where A, B and C are different lock classes.
+
+ NOTE: This document assumes that readers already understand how
+ lockdep works without crossrelease thus omits details. But there's
+ one thing to note. Lockdep pretends to pop a lock from held_locks
+ when releasing it. But it's subtly different from the original pop
+ operation because lockdep allows other than the top to be poped.
+
+In this case, lockdep adds 'the top of held_locks -> the lock to acquire'
+dependency every time acquiring a lock.
+
+After adding 'A -> B', a dependency graph will be:
+
+ A -> B
+
+ where A and B are different lock classes.
+
+And after adding 'B -> C', the graph will be:
+
+ A -> B -> C
+
+ where A, B and C are different lock classes.
+
+Let's performs commit step even for typical locks to add dependencies.
+Of course, commit step is not necessary for them, however, it would work
+well because this is a more general way.
+
+ acquire A
+ /*
+ * Queue A into hist_locks
+ *
+ * In hist_locks: A
+ * In graph: Empty
+ */
+
+ acquire B
+ /*
+ * Queue B into hist_locks
+ *
+ * In hist_locks: A, B
+ * In graph: Empty
+ */
+
+ acquire C
+ /*
+ * Queue C into hist_locks
+ *
+ * In hist_locks: A, B, C
+ * In graph: Empty
+ */
+
+ commit C
+ /*
+ * Add 'C -> ?'
+ * Answer the following to decide '?'
+ * What has been queued since acquire C: Nothing
+ *
+ * In hist_locks: A, B, C
+ * In graph: Empty
+ */
+
+ release C
+
+ commit B
+ /*
+ * Add 'B -> ?'
+ * Answer the following to decide '?'
+ * What has been queued since acquire B: C
+ *
+ * In hist_locks: A, B, C
+ * In graph: 'B -> C'
+ */
+
+ release B
+
+ commit A
+ /*
+ * Add 'A -> ?'
+ * Answer the following to decide '?'
+ * What has been queued since acquire A: B, C
+ *
+ * In hist_locks: A, B, C
+ * In graph: 'B -> C', 'A -> B', 'A -> C'
+ */
+
+ release A
+
+ where A, B and C are different lock classes.
+
+In this case, dependencies are added at the commit step as described.
+
+After commits for A, B and C, the graph will be:
+
+ A -> B -> C
+
+ where A, B and C are different lock classes.
+
+ NOTE: A dependency 'A -> C' is optimized out.
+
+We can see the former graph built without commit step is same as the
+latter graph built using commit steps. Of course the former way leads to
+earlier finish for building the graph, which means we can detect a
+deadlock or its possibility sooner. So the former way would be prefered
+when possible. But we cannot avoid using the latter way for crosslocks.
+
+Let's look at how commit steps work for crosslocks. In this case, the
+commit step is performed only on crosslock AX as real. And it assumes
+that the AX release context is different from the AX acquire context.
+
+ BX RELEASE CONTEXT BX ACQUIRE CONTEXT
+ ------------------ ------------------
+ acquire A
+ /*
+ * Push A onto held_locks
+ * Queue A into hist_locks
+ *
+ * In held_locks: A
+ * In hist_locks: A
+ * In graph: Empty
+ */
+
+ acquire BX
+ /*
+ * Add 'the top of held_locks -> BX'
+ *
+ * In held_locks: A
+ * In hist_locks: A
+ * In graph: 'A -> BX'
+ */
+
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ It must be guaranteed that the following operations are seen after
+ acquiring BX globally. It can be done by things like barrier.
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ acquire C
+ /*
+ * Push C onto held_locks
+ * Queue C into hist_locks
+ *
+ * In held_locks: C
+ * In hist_locks: C
+ * In graph: 'A -> BX'
+ */
+
+ release C
+ /*
+ * Pop C from held_locks
+ *
+ * In held_locks: Empty
+ * In hist_locks: C
+ * In graph: 'A -> BX'
+ */
+ acquire D
+ /*
+ * Push D onto held_locks
+ * Queue D into hist_locks
+ * Add 'the top of held_locks -> D'
+ *
+ * In held_locks: A, D
+ * In hist_locks: A, D
+ * In graph: 'A -> BX', 'A -> D'
+ */
+ acquire E
+ /*
+ * Push E onto held_locks
+ * Queue E into hist_locks
+ *
+ * In held_locks: E
+ * In hist_locks: C, E
+ * In graph: 'A -> BX', 'A -> D'
+ */
+
+ release E
+ /*
+ * Pop E from held_locks
+ *
+ * In held_locks: Empty
+ * In hist_locks: D, E
+ * In graph: 'A -> BX', 'A -> D'
+ */
+ release D
+ /*
+ * Pop D from held_locks
+ *
+ * In held_locks: A
+ * In hist_locks: A, D
+ * In graph: 'A -> BX', 'A -> D'
+ */
+ commit BX
+ /*
+ * Add 'BX -> ?'
+ * What has been queued since acquire BX: C, E
+ *
+ * In held_locks: Empty
+ * In hist_locks: D, E
+ * In graph: 'A -> BX', 'A -> D',
+ * 'BX -> C', 'BX -> E'
+ */
+
+ release BX
+ /*
+ * In held_locks: Empty
+ * In hist_locks: D, E
+ * In graph: 'A -> BX', 'A -> D',
+ * 'BX -> C', 'BX -> E'
+ */
+ release A
+ /*
+ * Pop A from held_locks
+ *
+ * In held_locks: Empty
+ * In hist_locks: A, D
+ * In graph: 'A -> BX', 'A -> D',
+ * 'BX -> C', 'BX -> E'
+ */
+
+ where A, BX, C,..., E are different lock classes, and a suffix 'X' is
+ added on crosslocks.
+
+Crossrelease considers all acquisitions after acqiuring BX are
+candidates which might create dependencies with BX. True dependencies
+will be determined when identifying the release context of BX. Meanwhile,
+all typical locks are queued so that they can be used at the commit step.
+And then two dependencies 'BX -> C' and 'BX -> E' are added at the
+commit step when identifying the release context.
+
+The final graph will be, with crossrelease:
+
+ -> C
+ /
+ -> BX -
+ / \
+ A - -> E
+ \
+ -> D
+
+ where A, BX, C,..., E are different lock classes, and a suffix 'X' is
+ added on crosslocks.
+
+However, the final graph will be, without crossrelease:
+
+ A -> D
+
+ where A and D are different lock classes.
+
+The former graph has three more dependencies, 'A -> BX', 'BX -> C' and
+'BX -> E' giving additional opportunities to check if they cause
+deadlocks. This way lockdep can detect a deadlock or its possibility
+caused by crosslocks.
+
+CONCLUSION
+
+We checked how crossrelease works with several examples.
+
+
+=============
+Optimizations
+=============
+
+Avoid duplication
+-----------------
+
+Crossrelease feature uses a cache like what lockdep already uses for
+dependency chains, but this time it's for caching CT type dependencies.
+Once that dependency is cached, the same will never be added again.
+
+
+Lockless for hot paths
+----------------------
+
+To keep all locks for later use at the commit step, crossrelease adopts
+a local array embedded in task_struct, which makes access to the data
+lockless by forcing it to happen only within the owner context. It's
+like how lockdep handles held_locks. Lockless implmentation is important
+since typical locks are very frequently acquired and released.
+
+
+=================================================
+APPENDIX A: What lockdep does to work aggresively
+=================================================
+
+A deadlock actually occurs when all wait operations creating circular
+dependencies run at the same time. Even though they don't, a potential
+deadlock exists if the problematic dependencies exist. Thus it's
+meaningful to detect not only an actual deadlock but also its potential
+possibility. The latter is rather valuable. When a deadlock occurs
+actually, we can identify what happens in the system by some means or
+other even without lockdep. However, there's no way to detect possiblity
+without lockdep unless the whole code is parsed in head. It's terrible.
+Lockdep does the both, and crossrelease only focuses on the latter.
+
+Whether or not a deadlock actually occurs depends on several factors.
+For example, what order contexts are switched in is a factor. Assuming
+circular dependencies exist, a deadlock would occur when contexts are
+switched so that all wait operations creating the dependencies run
+simultaneously. Thus to detect a deadlock possibility even in the case
+that it has not occured yet, lockdep should consider all possible
+combinations of dependencies, trying to:
+
+1. Use a global dependency graph.
+
+ Lockdep combines all dependencies into one global graph and uses them,
+ regardless of which context generates them or what order contexts are
+ switched in. Aggregated dependencies are only considered so they are
+ prone to be circular if a problem exists.
+
+2. Check dependencies between classes instead of instances.
+
+ What actually causes a deadlock are instances of lock. However,
+ lockdep checks dependencies between classes instead of instances.
+ This way lockdep can detect a deadlock which has not happened but
+ might happen in future by others but the same class.
+
+3. Assume all acquisitions lead to waiting.
+
+ Although locks might be acquired without waiting which is essential
+ to create dependencies, lockdep assumes all acquisitions lead to
+ waiting since it might be true some time or another.
+
+CONCLUSION
+
+Lockdep detects not only an actual deadlock but also its possibility,
+and the latter is more valuable.
+
+
+==================================================
+APPENDIX B: How to avoid adding false dependencies
+==================================================
+
+Remind what a dependency is. A dependency exists if:
+
+ 1. There are two waiters waiting for each event at a given time.
+ 2. The only way to wake up each waiter is to trigger its event.
+ 3. Whether one can be woken up depends on whether the other can.
+
+For example:
+
+ acquire A
+ acquire B /* A dependency 'A -> B' exists */
+ release B
+ release A
+
+ where A and B are different lock classes.
+
+A depedency 'A -> B' exists since:
+
+ 1. A waiter for A and a waiter for B might exist when acquiring B.
+ 2. Only way to wake up each is to release what it waits for.
+ 3. Whether the waiter for A can be woken up depends on whether the
+ other can. IOW, TASK X cannot release A if it fails to acquire B.
+
+For another example:
+
+ TASK X TASK Y
+ ------ ------
+ acquire AX
+ acquire B /* A dependency 'AX -> B' exists */
+ release B
+ release AX held by Y
+
+ where AX and B are different lock classes, and a suffix 'X' is added
+ on crosslocks.
+
+Even in this case involving crosslocks, the same rule can be applied. A
+depedency 'AX -> B' exists since:
+
+ 1. A waiter for AX and a waiter for B might exist when acquiring B.
+ 2. Only way to wake up each is to release what it waits for.
+ 3. Whether the waiter for AX can be woken up depends on whether the
+ other can. IOW, TASK X cannot release AX if it fails to acquire B.
+
+Let's take a look at more complicated example:
+
+ TASK X TASK Y
+ ------ ------
+ acquire B
+ release B
+ fork Y
+ acquire AX
+ acquire C /* A dependency 'AX -> C' exists */
+ release C
+ release AX held by Y
+
+ where AX, B and C are different lock classes, and a suffix 'X' is
+ added on crosslocks.
+
+Does a dependency 'AX -> B' exist? Nope.
+
+Two waiters are essential to create a dependency. However, waiters for
+AX and B to create 'AX -> B' cannot exist at the same time in this
+example. Thus the dependency 'AX -> B' cannot be created.
+
+It would be ideal if the full set of true ones can be considered. But
+we can ensure nothing but what actually happened. Relying on what
+actually happens at runtime, we can anyway add only true ones, though
+they might be a subset of true ones. It's similar to how lockdep works
+for typical locks. There might be more true dependencies than what
+lockdep has detected in runtime. Lockdep has no choice but to rely on
+what actually happens. Crossrelease also relies on it.
+
+CONCLUSION
+
+Relying on what actually happens, lockdep can avoid adding false
+dependencies.
diff --git a/include/linux/completion.h b/include/linux/completion.h
index 519e94915d18..0662a417febe 100644
--- a/include/linux/completion.h
+++ b/include/linux/completion.h
@@ -10,6 +10,9 @@
*/
#include <linux/wait.h>
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+#include <linux/lockdep.h>
+#endif
/*
* struct completion - structure used to maintain state for a "completion"
@@ -26,15 +29,58 @@
struct completion {
unsigned int done;
wait_queue_head_t wait;
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+ struct lockdep_map_cross map;
+#endif
};
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+static inline void complete_acquire(struct completion *x)
+{
+ lock_acquire_exclusive((struct lockdep_map *)&x->map, 0, 0, NULL, _RET_IP_);
+}
+
+static inline void complete_release(struct completion *x)
+{
+ lock_release((struct lockdep_map *)&x->map, 0, _RET_IP_);
+}
+
+static inline void complete_release_commit(struct completion *x)
+{
+ lock_commit_crosslock((struct lockdep_map *)&x->map);
+}
+
+#define init_completion_map(x, m) \
+do { \
+ lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map, \
+ (m)->name, (m)->key, 0); \
+ __init_completion(x); \
+} while (0)
+
+#define init_completion(x) \
+do { \
+ static struct lock_class_key __key; \
+ lockdep_init_map_crosslock((struct lockdep_map *)&(x)->map, \
+ "(completion)" #x, \
+ &__key, 0); \
+ __init_completion(x); \
+} while (0)
+#else
#define init_completion_map(x, m) __init_completion(x)
#define init_completion(x) __init_completion(x)
static inline void complete_acquire(struct completion *x) {}
static inline void complete_release(struct completion *x) {}
+static inline void complete_release_commit(struct completion *x) {}
+#endif
+#ifdef CONFIG_LOCKDEP_COMPLETIONS
+#define COMPLETION_INITIALIZER(work) \
+ { 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait), \
+ STATIC_CROSS_LOCKDEP_MAP_INIT("(completion)" #work, &(work)) }
+#else
#define COMPLETION_INITIALIZER(work) \
{ 0, __WAIT_QUEUE_HEAD_INITIALIZER((work).wait) }
+#endif
#define COMPLETION_INITIALIZER_ONSTACK_MAP(work, map) \
(*({ init_completion_map(&(work), &(map)); &(work); }))
diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 21619c92c377..bd1bda7248bf 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -38,18 +38,22 @@
# define trace_hardirq_enter() \
do { \
current->hardirq_context++; \
+ crossrelease_hist_start(XHLOCK_HARD); \
} while (0)
# define trace_hardirq_exit() \
do { \
current->hardirq_context--; \
+ crossrelease_hist_end(XHLOCK_HARD); \
} while (0)
# define lockdep_softirq_enter() \
do { \
current->softirq_context++; \
+ crossrelease_hist_start(XHLOCK_SOFT); \
} while (0)
# define lockdep_softirq_exit() \
do { \
current->softirq_context--; \
+ crossrelease_hist_end(XHLOCK_SOFT); \
} while (0)
#else
# define trace_hardirqs_on() do { } while (0)
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index b0d0b51c4d85..59b973ee4ce5 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -158,6 +158,12 @@ struct lockdep_map {
int cpu;
unsigned long ip;
#endif
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+ /*
+ * Whether it's a crosslock.
+ */
+ int cross;
+#endif
};
static inline void lockdep_copy_map(struct lockdep_map *to,
@@ -261,8 +267,95 @@ struct held_lock {
unsigned int hardirqs_off:1;
unsigned int references:12; /* 32 bits */
unsigned int pin_count;
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+ /*
+ * Generation id.
+ *
+ * A value of cross_gen_id will be stored when holding this,
+ * which is globally increased whenever each crosslock is held.
+ */
+ unsigned int gen_id;
+#endif
+};
+
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#define MAX_XHLOCK_TRACE_ENTRIES 5
+
+/*
+ * This is for keeping locks waiting for commit so that true dependencies
+ * can be added at commit step.
+ */
+struct hist_lock {
+ /*
+ * Id for each entry in the ring buffer. This is used to
+ * decide whether the ring buffer was overwritten or not.
+ *
+ * For example,
+ *
+ * |<----------- hist_lock ring buffer size ------->|
+ * pppppppppppppppppppppiiiiiiiiiiiiiiiiiiiiiiiiiiiii
+ * wrapped > iiiiiiiiiiiiiiiiiiiiiiiiiii.......................
+ *
+ * where 'p' represents an acquisition in process
+ * context, 'i' represents an acquisition in irq
+ * context.
+ *
+ * In this example, the ring buffer was overwritten by
+ * acquisitions in irq context, that should be detected on
+ * rollback or commit.
+ */
+ unsigned int hist_id;
+
+ /*
+ * Seperate stack_trace data. This will be used at commit step.
+ */
+ struct stack_trace trace;
+ unsigned long trace_entries[MAX_XHLOCK_TRACE_ENTRIES];
+
+ /*
+ * Seperate hlock instance. This will be used at commit step.
+ *
+ * TODO: Use a smaller data structure containing only necessary
+ * data. However, we should make lockdep code able to handle the
+ * smaller one first.
+ */
+ struct held_lock hlock;
};
+/*
+ * To initialize a lock as crosslock, lockdep_init_map_crosslock() should
+ * be called instead of lockdep_init_map().
+ */
+struct cross_lock {
+ /*
+ * When more than one acquisition of crosslocks are overlapped,
+ * we have to perform commit for them based on cross_gen_id of
+ * the first acquisition, which allows us to add more true
+ * dependencies.
+ *
+ * Moreover, when no acquisition of a crosslock is in progress,
+ * we should not perform commit because the lock might not exist
+ * any more, which might cause incorrect memory access. So we
+ * have to track the number of acquisitions of a crosslock.
+ */
+ int nr_acquire;
+
+ /*
+ * Seperate hlock instance. This will be used at commit step.
+ *
+ * TODO: Use a smaller data structure containing only necessary
+ * data. However, we should make lockdep code able to handle the
+ * smaller one first.
+ */
+ struct held_lock hlock;
+};
+
+struct lockdep_map_cross {
+ struct lockdep_map map;
+ struct cross_lock xlock;
+};
+#endif
+
/*
* Initialization, self-test and debugging-output methods:
*/
@@ -464,6 +557,37 @@ enum xhlock_context_t {
XHLOCK_CTX_NR,
};
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+extern void lockdep_init_map_crosslock(struct lockdep_map *lock,
+ const char *name,
+ struct lock_class_key *key,
+ int subclass);
+extern void lock_commit_crosslock(struct lockdep_map *lock);
+
+/*
+ * What we essencially have to initialize is 'nr_acquire'. Other members
+ * will be initialized in add_xlock().
+ */
+#define STATIC_CROSS_LOCK_INIT() \
+ { .nr_acquire = 0,}
+
+#define STATIC_CROSS_LOCKDEP_MAP_INIT(_name, _key) \
+ { .map.name = (_name), .map.key = (void *)(_key), \
+ .map.cross = 1, .xlock = STATIC_CROSS_LOCK_INIT(), }
+
+/*
+ * To initialize a lockdep_map statically use this macro.
+ * Note that _name must not be NULL.
+ */
+#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
+ { .name = (_name), .key = (void *)(_key), .cross = 0, }
+
+extern void crossrelease_hist_start(enum xhlock_context_t c);
+extern void crossrelease_hist_end(enum xhlock_context_t c);
+extern void lockdep_invariant_state(bool force);
+extern void lockdep_init_task(struct task_struct *task);
+extern void lockdep_free_task(struct task_struct *task);
+#else /* !CROSSRELEASE */
#define lockdep_init_map_crosslock(m, n, k, s) do {} while (0)
/*
* To initialize a lockdep_map statically use this macro.
@@ -472,9 +596,12 @@ enum xhlock_context_t {
#define STATIC_LOCKDEP_MAP_INIT(_name, _key) \
{ .name = (_name), .key = (void *)(_key), }
+static inline void crossrelease_hist_start(enum xhlock_context_t c) {}
+static inline void crossrelease_hist_end(enum xhlock_context_t c) {}
static inline void lockdep_invariant_state(bool force) {}
static inline void lockdep_init_task(struct task_struct *task) {}
static inline void lockdep_free_task(struct task_struct *task) {}
+#endif /* CROSSRELEASE */
#ifdef CONFIG_LOCK_STAT
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 977cb57d7bc9..155c7690eb15 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -936,6 +936,17 @@ struct task_struct {
struct held_lock held_locks[MAX_LOCK_DEPTH];
#endif
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#define MAX_XHLOCKS_NR 64UL
+ struct hist_lock *xhlocks; /* Crossrelease history locks */
+ unsigned int xhlock_idx;
+ /* For restoring at history boundaries */
+ unsigned int xhlock_idx_hist[XHLOCK_CTX_NR];
+ unsigned int hist_id;
+ /* For overwrite check at each context exit */
+ unsigned int hist_id_save[XHLOCK_CTX_NR];
+#endif
+
#ifdef CONFIG_UBSAN
unsigned int in_ubsan;
#endif
diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index dd13f865ad40..c4d0fe3538da 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -58,6 +58,10 @@
#define CREATE_TRACE_POINTS
#include <trace/events/lock.h>
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+#include <linux/slab.h>
+#endif
+
#ifdef CONFIG_PROVE_LOCKING
int prove_locking = 1;
module_param(prove_locking, int, 0644);
@@ -72,6 +76,19 @@ module_param(lock_stat, int, 0644);
#define lock_stat 0
#endif
+#ifdef CONFIG_BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
+static int crossrelease_fullstack = 1;
+#else
+static int crossrelease_fullstack;
+#endif
+static int __init allow_crossrelease_fullstack(char *str)
+{
+ crossrelease_fullstack = 1;
+ return 0;
+}
+
+early_param("crossrelease_fullstack", allow_crossrelease_fullstack);
+
/*
* lockdep_lock: protects the lockdep graph, the hashes and the
* class/list/hash allocators.
@@ -731,6 +748,18 @@ static bool assign_lock_key(struct lockdep_map *lock)
return true;
}
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+static void cross_init(struct lockdep_map *lock, int cross);
+static int cross_lock(struct lockdep_map *lock);
+static int lock_acquire_crosslock(struct held_lock *hlock);
+static int lock_release_crosslock(struct lockdep_map *lock);
+#else
+static inline void cross_init(struct lockdep_map *lock, int cross) {}
+static inline int cross_lock(struct lockdep_map *lock) { return 0; }
+static inline int lock_acquire_crosslock(struct held_lock *hlock) { return 2; }
+static inline int lock_release_crosslock(struct lockdep_map *lock) { return 2; }
+#endif
+
/*
* Register a lock's class in the hash-table, if the class is not present
* yet. Otherwise we look it up. We cache the result in the lock object
@@ -1125,22 +1154,41 @@ print_circular_lock_scenario(struct held_lock *src,
printk(KERN_CONT "\n\n");
}
- printk(" Possible unsafe locking scenario:\n\n");
- printk(" CPU0 CPU1\n");
- printk(" ---- ----\n");
- printk(" lock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(parent);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(target);
- printk(KERN_CONT ");\n");
- printk(" lock(");
- __print_lock_name(source);
- printk(KERN_CONT ");\n");
- printk("\n *** DEADLOCK ***\n\n");
+ if (cross_lock(tgt->instance)) {
+ printk(" Possible unsafe locking scenario by crosslock:\n\n");
+ printk(" CPU0 CPU1\n");
+ printk(" ---- ----\n");
+ printk(" lock(");
+ __print_lock_name(parent);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(source);
+ printk(KERN_CONT ");\n");
+ printk(" unlock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk("\n *** DEADLOCK ***\n\n");
+ } else {
+ printk(" Possible unsafe locking scenario:\n\n");
+ printk(" CPU0 CPU1\n");
+ printk(" ---- ----\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(parent);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(target);
+ printk(KERN_CONT ");\n");
+ printk(" lock(");
+ __print_lock_name(source);
+ printk(KERN_CONT ");\n");
+ printk("\n *** DEADLOCK ***\n\n");
+ }
}
/*
@@ -1166,7 +1214,10 @@ print_circular_bug_header(struct lock_list *entry, unsigned int depth,
curr->comm, task_pid_nr(curr));
print_lock(check_src);
- pr_warn("\nbut task is already holding lock:\n");
+ if (cross_lock(check_tgt->instance))
+ pr_warn("\nbut now in release context of a crosslock acquired at the following:\n");
+ else
+ pr_warn("\nbut task is already holding lock:\n");
print_lock(check_tgt);
pr_warn("\nwhich lock already depends on the new lock.\n\n");
@@ -1196,7 +1247,9 @@ static noinline int print_circular_bug(struct lock_list *this,
if (!debug_locks_off_graph_unlock() || debug_locks_silent)
return 0;
- if (!save_trace(&this->trace))
+ if (cross_lock(check_tgt->instance))
+ this->trace = *trace;
+ else if (!save_trace(&this->trace))
return 0;
depth = get_lock_depth(target);
@@ -1800,6 +1853,9 @@ check_deadlock(struct task_struct *curr, struct held_lock *next,
if (nest)
return 2;
+ if (cross_lock(prev->instance))
+ continue;
+
return print_deadlock_bug(curr, prev, next);
}
return 1;
@@ -1965,26 +2021,31 @@ check_prevs_add(struct task_struct *curr, struct held_lock *next)
for (;;) {
int distance = curr->lockdep_depth - depth + 1;
hlock = curr->held_locks + depth - 1;
-
/*
- * Only non-recursive-read entries get new dependencies
- * added:
+ * Only non-crosslock entries get new dependencies added.
+ * Crosslock entries will be added by commit later:
*/
- if (hlock->read != 2 && hlock->check) {
- int ret = check_prev_add(curr, hlock, next, distance, &trace, save_trace);
- if (!ret)
- return 0;
-
+ if (!cross_lock(hlock->instance)) {
/*
- * Stop after the first non-trylock entry,
- * as non-trylock entries have added their
- * own direct dependencies already, so this
- * lock is connected to them indirectly:
+ * Only non-recursive-read entries get new dependencies
+ * added:
*/
- if (!hlock->trylock)
- break;
- }
+ if (hlock->read != 2 && hlock->check) {
+ int ret = check_prev_add(curr, hlock, next,
+ distance, &trace, save_trace);
+ if (!ret)
+ return 0;
+ /*
+ * Stop after the first non-trylock entry,
+ * as non-trylock entries have added their
+ * own direct dependencies already, so this
+ * lock is connected to them indirectly:
+ */
+ if (!hlock->trylock)
+ break;
+ }
+ }
depth--;
/*
* End of lock-stack?
@@ -3216,10 +3277,21 @@ static void __lockdep_init_map(struct lockdep_map *lock, const char *name,
void lockdep_init_map(struct lockdep_map *lock, const char *name,
struct lock_class_key *key, int subclass)
{
+ cross_init(lock, 0);
__lockdep_init_map(lock, name, key, subclass);
}
EXPORT_SYMBOL_GPL(lockdep_init_map);
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+void lockdep_init_map_crosslock(struct lockdep_map *lock, const char *name,
+ struct lock_class_key *key, int subclass)
+{
+ cross_init(lock, 1);
+ __lockdep_init_map(lock, name, key, subclass);
+}
+EXPORT_SYMBOL_GPL(lockdep_init_map_crosslock);
+#endif
+
struct lock_class_key __lockdep_no_validate__;
EXPORT_SYMBOL_GPL(__lockdep_no_validate__);
@@ -3275,6 +3347,7 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
int chain_head = 0;
int class_idx;
u64 chain_key;
+ int ret;
if (unlikely(!debug_locks))
return 0;
@@ -3323,7 +3396,8 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
class_idx = class - lock_classes + 1;
- if (depth) {
+ /* TODO: nest_lock is not implemented for crosslock yet. */
+ if (depth && !cross_lock(lock)) {
hlock = curr->held_locks + depth - 1;
if (hlock->class_idx == class_idx && nest_lock) {
if (hlock->references) {
@@ -3411,6 +3485,14 @@ static int __lock_acquire(struct lockdep_map *lock, unsigned int subclass,
if (!validate_chain(curr, lock, hlock, chain_head, chain_key))
return 0;
+ ret = lock_acquire_crosslock(hlock);
+ /*
+ * 2 means normal acquire operations are needed. Otherwise, it's
+ * ok just to return with '0:fail, 1:success'.
+ */
+ if (ret != 2)
+ return ret;
+
curr->curr_chain_key = chain_key;
curr->lockdep_depth++;
check_chain_key(curr);
@@ -3649,11 +3731,19 @@ __lock_release(struct lockdep_map *lock, int nested, unsigned long ip)
struct task_struct *curr = current;
struct held_lock *hlock;
unsigned int depth;
- int i;
+ int ret, i;
if (unlikely(!debug_locks))
return 0;
+ ret = lock_release_crosslock(lock);
+ /*
+ * 2 means normal release operations are needed. Otherwise, it's
+ * ok just to return with '0:fail, 1:success'.
+ */
+ if (ret != 2)
+ return ret;
+
depth = curr->lockdep_depth;
/*
* So we're all set to release this lock.. wait what lock? We don't
@@ -4536,3 +4626,495 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
dump_stack();
}
EXPORT_SYMBOL_GPL(lockdep_rcu_suspicious);
+
+#ifdef CONFIG_LOCKDEP_CROSSRELEASE
+
+/*
+ * Crossrelease works by recording a lock history for each thread and
+ * connecting those historic locks that were taken after the
+ * wait_for_completion() in the complete() context.
+ *
+ * Task-A Task-B
+ *
+ * mutex_lock(&A);
+ * mutex_unlock(&A);
+ *
+ * wait_for_completion(&C);
+ * lock_acquire_crosslock();
+ * atomic_inc_return(&cross_gen_id);
+ * |
+ * | mutex_lock(&B);
+ * | mutex_unlock(&B);
+ * |
+ * | complete(&C);
+ * `-- lock_commit_crosslock();
+ *
+ * Which will then add a dependency between B and C.
+ */
+
+#define xhlock(i) (current->xhlocks[(i) % MAX_XHLOCKS_NR])
+
+/*
+ * Whenever a crosslock is held, cross_gen_id will be increased.
+ */
+static atomic_t cross_gen_id; /* Can be wrapped */
+
+/*
+ * Make an entry of the ring buffer invalid.
+ */
+static inline void invalidate_xhlock(struct hist_lock *xhlock)
+{
+ /*
+ * Normally, xhlock->hlock.instance must be !NULL.
+ */
+ xhlock->hlock.instance = NULL;
+}
+
+/*
+ * Lock history stacks; we have 2 nested lock history stacks:
+ *
+ * HARD(IRQ)
+ * SOFT(IRQ)
+ *
+ * The thing is that once we complete a HARD/SOFT IRQ the future task locks
+ * should not depend on any of the locks observed while running the IRQ. So
+ * what we do is rewind the history buffer and erase all our knowledge of that
+ * temporal event.
+ */
+
+void crossrelease_hist_start(enum xhlock_context_t c)
+{
+ struct task_struct *cur = current;
+
+ if (!cur->xhlocks)
+ return;
+
+ cur->xhlock_idx_hist[c] = cur->xhlock_idx;
+ cur->hist_id_save[c] = cur->hist_id;
+}
+
+void crossrelease_hist_end(enum xhlock_context_t c)
+{
+ struct task_struct *cur = current;
+
+ if (cur->xhlocks) {
+ unsigned int idx = cur->xhlock_idx_hist[c];
+ struct hist_lock *h = &xhlock(idx);
+
+ cur->xhlock_idx = idx;
+
+ /* Check if the ring was overwritten. */
+ if (h->hist_id != cur->hist_id_save[c])
+ invalidate_xhlock(h);
+ }
+}
+
+/*
+ * lockdep_invariant_state() is used to annotate independence inside a task, to
+ * make one task look like multiple independent 'tasks'.
+ *
+ * Take for instance workqueues; each work is independent of the last. The
+ * completion of a future work does not depend on the completion of a past work
+ * (in general). Therefore we must not carry that (lock) dependency across
+ * works.
+ *
+ * This is true for many things; pretty much all kthreads fall into this
+ * pattern, where they have an invariant state and future completions do not
+ * depend on past completions. Its just that since they all have the 'same'
+ * form -- the kthread does the same over and over -- it doesn't typically
+ * matter.
+ *
+ * The same is true for system-calls, once a system call is completed (we've
+ * returned to userspace) the next system call does not depend on the lock
+ * history of the previous system call.
+ *
+ * They key property for independence, this invariant state, is that it must be
+ * a point where we hold no locks and have no history. Because if we were to
+ * hold locks, the restore at _end() would not necessarily recover it's history
+ * entry. Similarly, independence per-definition means it does not depend on
+ * prior state.
+ */
+void lockdep_invariant_state(bool force)
+{
+ /*
+ * We call this at an invariant point, no current state, no history.
+ * Verify the former, enforce the latter.
+ */
+ WARN_ON_ONCE(!force && current->lockdep_depth);
+ if (current->xhlocks)
+ invalidate_xhlock(&xhlock(current->xhlock_idx));
+}
+
+static int cross_lock(struct lockdep_map *lock)
+{
+ return lock ? lock->cross : 0;
+}
+
+/*
+ * This is needed to decide the relationship between wrapable variables.
+ */
+static inline int before(unsigned int a, unsigned int b)
+{
+ return (int)(a - b) < 0;
+}
+
+static inline struct lock_class *xhlock_class(struct hist_lock *xhlock)
+{
+ return hlock_class(&xhlock->hlock);
+}
+
+static inline struct lock_class *xlock_class(struct cross_lock *xlock)
+{
+ return hlock_class(&xlock->hlock);
+}
+
+/*
+ * Should we check a dependency with previous one?
+ */
+static inline int depend_before(struct held_lock *hlock)
+{
+ return hlock->read != 2 && hlock->check && !hlock->trylock;
+}
+
+/*
+ * Should we check a dependency with next one?
+ */
+static inline int depend_after(struct held_lock *hlock)
+{
+ return hlock->read != 2 && hlock->check;
+}
+
+/*
+ * Check if the xhlock is valid, which would be false if,
+ *
+ * 1. Has not used after initializaion yet.
+ * 2. Got invalidated.
+ *
+ * Remind hist_lock is implemented as a ring buffer.
+ */
+static inline int xhlock_valid(struct hist_lock *xhlock)
+{
+ /*
+ * xhlock->hlock.instance must be !NULL.
+ */
+ return !!xhlock->hlock.instance;
+}
+
+/*
+ * Record a hist_lock entry.
+ *
+ * Irq disable is only required.
+ */
+static void add_xhlock(struct held_lock *hlock)
+{
+ unsigned int idx = ++current->xhlock_idx;
+ struct hist_lock *xhlock = &xhlock(idx);
+
+#ifdef CONFIG_DEBUG_LOCKDEP
+ /*
+ * This can be done locklessly because they are all task-local
+ * state, we must however ensure IRQs are disabled.
+ */
+ WARN_ON_ONCE(!irqs_disabled());
+#endif
+
+ /* Initialize hist_lock's members */
+ xhlock->hlock = *hlock;
+ xhlock->hist_id = ++current->hist_id;
+
+ xhlock->trace.nr_entries = 0;
+ xhlock->trace.max_entries = MAX_XHLOCK_TRACE_ENTRIES;
+ xhlock->trace.entries = xhlock->trace_entries;
+
+ if (crossrelease_fullstack) {
+ xhlock->trace.skip = 3;
+ save_stack_trace(&xhlock->trace);
+ } else {
+ xhlock->trace.nr_entries = 1;
+ xhlock->trace.entries[0] = hlock->acquire_ip;
+ }
+}
+
+static inline int same_context_xhlock(struct hist_lock *xhlock)
+{
+ return xhlock->hlock.irq_context == task_irq_context(current);
+}
+
+/*
+ * This should be lockless as far as possible because this would be
+ * called very frequently.
+ */
+static void check_add_xhlock(struct held_lock *hlock)
+{
+ /*
+ * Record a hist_lock, only in case that acquisitions ahead
+ * could depend on the held_lock. For example, if the held_lock
+ * is trylock then acquisitions ahead never depends on that.
+ * In that case, we don't need to record it. Just return.
+ */
+ if (!current->xhlocks || !depend_before(hlock))
+ return;
+
+ add_xhlock(hlock);
+}
+
+/*
+ * For crosslock.
+ */
+static int add_xlock(struct held_lock *hlock)
+{
+ struct cross_lock *xlock;
+ unsigned int gen_id;
+
+ if (!graph_lock())
+ return 0;
+
+ xlock = &((struct lockdep_map_cross *)hlock->instance)->xlock;
+
+ /*
+ * When acquisitions for a crosslock are overlapped, we use
+ * nr_acquire to perform commit for them, based on cross_gen_id
+ * of the first acquisition, which allows to add additional
+ * dependencies.
+ *
+ * Moreover, when no acquisition of a crosslock is in progress,
+ * we should not perform commit because the lock might not exist
+ * any more, which might cause incorrect memory access. So we
+ * have to track the number of acquisitions of a crosslock.
+ *
+ * depend_after() is necessary to initialize only the first
+ * valid xlock so that the xlock can be used on its commit.
+ */
+ if (xlock->nr_acquire++ && depend_after(&xlock->hlock))
+ goto unlock;
+
+ gen_id = (unsigned int)atomic_inc_return(&cross_gen_id);
+ xlock->hlock = *hlock;
+ xlock->hlock.gen_id = gen_id;
+unlock:
+ graph_unlock();
+ return 1;
+}
+
+/*
+ * Called for both normal and crosslock acquires. Normal locks will be
+ * pushed on the hist_lock queue. Cross locks will record state and
+ * stop regular lock_acquire() to avoid being placed on the held_lock
+ * stack.
+ *
+ * Return: 0 - failure;
+ * 1 - crosslock, done;
+ * 2 - normal lock, continue to held_lock[] ops.
+ */
+static int lock_acquire_crosslock(struct held_lock *hlock)
+{
+ /*
+ * CONTEXT 1 CONTEXT 2
+ * --------- ---------
+ * lock A (cross)
+ * X = atomic_inc_return(&cross_gen_id)
+ * ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ * Y = atomic_read_acquire(&cross_gen_id)
+ * lock B
+ *
+ * atomic_read_acquire() is for ordering between A and B,
+ * IOW, A happens before B, when CONTEXT 2 see Y >= X.
+ *
+ * Pairs with atomic_inc_return() in add_xlock().
+ */
+ hlock->gen_id = (unsigned int)atomic_read_acquire(&cross_gen_id);
+
+ if (cross_lock(hlock->instance))
+ return add_xlock(hlock);
+
+ check_add_xhlock(hlock);
+ return 2;
+}
+
+static int copy_trace(struct stack_trace *trace)
+{
+ unsigned long *buf = stack_trace + nr_stack_trace_entries;
+ unsigned int max_nr = MAX_STACK_TRACE_ENTRIES - nr_stack_trace_entries;
+ unsigned int nr = min(max_nr, trace->nr_entries);
+
+ trace->nr_entries = nr;
+ memcpy(buf, trace->entries, nr * sizeof(trace->entries[0]));
+ trace->entries = buf;
+ nr_stack_trace_entries += nr;
+
+ if (nr_stack_trace_entries >= MAX_STACK_TRACE_ENTRIES-1) {
+ if (!debug_locks_off_graph_unlock())
+ return 0;
+
+ print_lockdep_off("BUG: MAX_STACK_TRACE_ENTRIES too low!");
+ dump_stack();
+
+ return 0;
+ }
+
+ return 1;
+}
+
+static int commit_xhlock(struct cross_lock *xlock, struct hist_lock *xhlock)
+{
+ unsigned int xid, pid;
+ u64 chain_key;
+
+ xid = xlock_class(xlock) - lock_classes;
+ chain_key = iterate_chain_key((u64)0, xid);
+ pid = xhlock_class(xhlock) - lock_classes;
+ chain_key = iterate_chain_key(chain_key, pid);
+
+ if (lookup_chain_cache(chain_key))
+ return 1;
+
+ if (!add_chain_cache_classes(xid, pid, xhlock->hlock.irq_context,
+ chain_key))
+ return 0;
+
+ if (!check_prev_add(current, &xlock->hlock, &xhlock->hlock, 1,
+ &xhlock->trace, copy_trace))
+ return 0;
+
+ return 1;
+}
+
+static void commit_xhlocks(struct cross_lock *xlock)
+{
+ unsigned int cur = current->xhlock_idx;
+ unsigned int prev_hist_id = xhlock(cur).hist_id;
+ unsigned int i;
+
+ if (!graph_lock())
+ return;
+
+ if (xlock->nr_acquire) {
+ for (i = 0; i < MAX_XHLOCKS_NR; i++) {
+ struct hist_lock *xhlock = &xhlock(cur - i);
+
+ if (!xhlock_valid(xhlock))
+ break;
+
+ if (before(xhlock->hlock.gen_id, xlock->hlock.gen_id))
+ break;
+
+ if (!same_context_xhlock(xhlock))
+ break;
+
+ /*
+ * Filter out the cases where the ring buffer was
+ * overwritten and the current entry has a bigger
+ * hist_id than the previous one, which is impossible
+ * otherwise:
+ */
+ if (unlikely(before(prev_hist_id, xhlock->hist_id)))
+ break;
+
+ prev_hist_id = xhlock->hist_id;
+
+ /*
+ * commit_xhlock() returns 0 with graph_lock already
+ * released if fail.
+ */
+ if (!commit_xhlock(xlock, xhlock))
+ return;
+ }
+ }
+
+ graph_unlock();
+}
+
+void lock_commit_crosslock(struct lockdep_map *lock)
+{
+ struct cross_lock *xlock;
+ unsigned long flags;
+
+ if (unlikely(!debug_locks || current->lockdep_recursion))
+ return;
+
+ if (!current->xhlocks)
+ return;
+
+ /*
+ * Do commit hist_locks with the cross_lock, only in case that
+ * the cross_lock could depend on acquisitions after that.
+ *
+ * For example, if the cross_lock does not have the 'check' flag
+ * then we don't need to check dependencies and commit for that.
+ * Just skip it. In that case, of course, the cross_lock does
+ * not depend on acquisitions ahead, either.
+ *
+ * WARNING: Don't do that in add_xlock() in advance. When an
+ * acquisition context is different from the commit context,
+ * invalid(skipped) cross_lock might be accessed.
+ */
+ if (!depend_after(&((struct lockdep_map_cross *)lock)->xlock.hlock))
+ return;
+
+ raw_local_irq_save(flags);
+ check_flags(flags);
+ current->lockdep_recursion = 1;
+ xlock = &((struct lockdep_map_cross *)lock)->xlock;
+ commit_xhlocks(xlock);
+ current->lockdep_recursion = 0;
+ raw_local_irq_restore(flags);
+}
+EXPORT_SYMBOL_GPL(lock_commit_crosslock);
+
+/*
+ * Return: 0 - failure;
+ * 1 - crosslock, done;
+ * 2 - normal lock, continue to held_lock[] ops.
+ */
+static int lock_release_crosslock(struct lockdep_map *lock)
+{
+ if (cross_lock(lock)) {
+ if (!graph_lock())
+ return 0;
+ ((struct lockdep_map_cross *)lock)->xlock.nr_acquire--;
+ graph_unlock();
+ return 1;
+ }
+ return 2;
+}
+
+static void cross_init(struct lockdep_map *lock, int cross)
+{
+ if (cross)
+ ((struct lockdep_map_cross *)lock)->xlock.nr_acquire = 0;
+
+ lock->cross = cross;
+
+ /*
+ * Crossrelease assumes that the ring buffer size of xhlocks
+ * is aligned with power of 2. So force it on build.
+ */
+ BUILD_BUG_ON(MAX_XHLOCKS_NR & (MAX_XHLOCKS_NR - 1));
+}
+
+void lockdep_init_task(struct task_struct *task)
+{
+ int i;
+
+ task->xhlock_idx = UINT_MAX;
+ task->hist_id = 0;
+
+ for (i = 0; i < XHLOCK_CTX_NR; i++) {
+ task->xhlock_idx_hist[i] = UINT_MAX;
+ task->hist_id_save[i] = 0;
+ }
+
+ task->xhlocks = kzalloc(sizeof(struct hist_lock) * MAX_XHLOCKS_NR,
+ GFP_KERNEL);
+}
+
+void lockdep_free_task(struct task_struct *task)
+{
+ if (task->xhlocks) {
+ void *tmp = task->xhlocks;
+ /* Diable crossrelease for current */
+ task->xhlocks = NULL;
+ kfree(tmp);
+ }
+}
+#endif
diff --git a/kernel/sched/completion.c b/kernel/sched/completion.c
index a1ad5b7d5521..0f7c442f9dd7 100644
--- a/kernel/sched/completion.c
+++ b/kernel/sched/completion.c
@@ -31,6 +31,11 @@ void complete(struct completion *x)
spin_lock_irqsave(&x->wait.lock, flags);
+ /*
+ * Perform commit of crossrelease here.
+ */
+ complete_release_commit(x);
+
if (x->done != UINT_MAX)
x->done++;
__wake_up_locked(&x->wait, TASK_NORMAL, 1);
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 1ead06829fdb..fbfdf7c263d2 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1055,6 +1055,8 @@ config PROVE_LOCKING
select DEBUG_RWSEMS if RWSEM_SPIN_ON_OWNER
select DEBUG_WW_MUTEX_SLOWPATH
select DEBUG_LOCK_ALLOC
+ select LOCKDEP_CROSSRELEASE
+ select LOCKDEP_COMPLETIONS
select TRACE_IRQFLAGS
default n
help
@@ -1187,6 +1189,37 @@ config LOCKDEP
config LOCKDEP_SMALL
bool
+config LOCKDEP_CROSSRELEASE
+ bool
+ help
+ This makes lockdep work for crosslock which is a lock allowed to
+ be released in a different context from the acquisition context.
+ Normally a lock must be released in the context acquiring the lock.
+ However, relexing this constraint helps synchronization primitives
+ such as page locks or completions can use the lock correctness
+ detector, lockdep.
+
+config LOCKDEP_COMPLETIONS
+ bool
+ help
+ A deadlock caused by wait_for_completion() and complete() can be
+ detected by lockdep using crossrelease feature.
+
+config BOOTPARAM_LOCKDEP_CROSSRELEASE_FULLSTACK
+ bool "Enable the boot parameter, crossrelease_fullstack"
+ depends on LOCKDEP_CROSSRELEASE
+ default n
+ help
+ The lockdep "cross-release" feature needs to record stack traces
+ (of calling functions) for all acquisitions, for eventual later
+ use during analysis. By default only a single caller is recorded,
+ because the unwind operation can be very expensive with deeper
+ stack chains.
+
+ However a boot parameter, crossrelease_fullstack, was
+ introduced since sometimes deeper traces are required for full
+ analysis. This option turns on the boot parameter.
+
config DEBUG_LOCKDEP
bool "Lock dependency engine debugging"
depends on DEBUG_KERNEL && LOCKDEP
--
2.19.1
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