[PATCH 01/25] dma-fence: basic lockdep annotations
Daniel Vetter
daniel at ffwll.ch
Mon Jul 13 16:26:10 UTC 2020
Hi Christian,
On Wed, Jul 08, 2020 at 04:57:21PM +0200, Christian König wrote:
> Could we merge this controlled by a separate config option?
>
> This way we could have the checks upstream without having to fix all the
> stuff before we do this?
Discussions died out a bit, do you consider this a blocker for the first
two patches, or good for an ack on these?
Like I said I don't plan to merge patches where I know it causes a lockdep
splat with a driver still. At least for now.
Thanks, Daniel
>
> Thanks,
> Christian.
>
> Am 07.07.20 um 22:12 schrieb Daniel Vetter:
> > Design is similar to the lockdep annotations for workers, but with
> > some twists:
> >
> > - We use a read-lock for the execution/worker/completion side, so that
> > this explicit annotation can be more liberally sprinkled around.
> > With read locks lockdep isn't going to complain if the read-side
> > isn't nested the same way under all circumstances, so ABBA deadlocks
> > are ok. Which they are, since this is an annotation only.
> >
> > - We're using non-recursive lockdep read lock mode, since in recursive
> > read lock mode lockdep does not catch read side hazards. And we
> > _very_ much want read side hazards to be caught. For full details of
> > this limitation see
> >
> > commit e91498589746065e3ae95d9a00b068e525eec34f
> > Author: Peter Zijlstra <peterz at infradead.org>
> > Date: Wed Aug 23 13:13:11 2017 +0200
> >
> > locking/lockdep/selftests: Add mixed read-write ABBA tests
> >
> > - To allow nesting of the read-side explicit annotations we explicitly
> > keep track of the nesting. lock_is_held() allows us to do that.
> >
> > - The wait-side annotation is a write lock, and entirely done within
> > dma_fence_wait() for everyone by default.
> >
> > - To be able to freely annotate helper functions I want to make it ok
> > to call dma_fence_begin/end_signalling from soft/hardirq context.
> > First attempt was using the hardirq locking context for the write
> > side in lockdep, but this forces all normal spinlocks nested within
> > dma_fence_begin/end_signalling to be spinlocks. That bollocks.
> >
> > The approach now is to simple check in_atomic(), and for these cases
> > entirely rely on the might_sleep() check in dma_fence_wait(). That
> > will catch any wrong nesting against spinlocks from soft/hardirq
> > contexts.
> >
> > The idea here is that every code path that's critical for eventually
> > signalling a dma_fence should be annotated with
> > dma_fence_begin/end_signalling. The annotation ideally starts right
> > after a dma_fence is published (added to a dma_resv, exposed as a
> > sync_file fd, attached to a drm_syncobj fd, or anything else that
> > makes the dma_fence visible to other kernel threads), up to and
> > including the dma_fence_wait(). Examples are irq handlers, the
> > scheduler rt threads, the tail of execbuf (after the corresponding
> > fences are visible), any workers that end up signalling dma_fences and
> > really anything else. Not annotated should be code paths that only
> > complete fences opportunistically as the gpu progresses, like e.g.
> > shrinker/eviction code.
> >
> > The main class of deadlocks this is supposed to catch are:
> >
> > Thread A:
> >
> > mutex_lock(A);
> > mutex_unlock(A);
> >
> > dma_fence_signal();
> >
> > Thread B:
> >
> > mutex_lock(A);
> > dma_fence_wait();
> > mutex_unlock(A);
> >
> > Thread B is blocked on A signalling the fence, but A never gets around
> > to that because it cannot acquire the lock A.
> >
> > Note that dma_fence_wait() is allowed to be nested within
> > dma_fence_begin/end_signalling sections. To allow this to happen the
> > read lock needs to be upgraded to a write lock, which means that any
> > other lock is acquired between the dma_fence_begin_signalling() call and
> > the call to dma_fence_wait(), and still held, this will result in an
> > immediate lockdep complaint. The only other option would be to not
> > annotate such calls, defeating the point. Therefore these annotations
> > cannot be sprinkled over the code entirely mindless to avoid false
> > positives.
> >
> > Originally I hope that the cross-release lockdep extensions would
> > alleviate the need for explicit annotations:
> >
> > https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.net%2FArticles%2F709849%2F&data=02%7C01%7Cchristian.koenig%40amd.com%7Cff1a9dd17c544534eeb808d822b21ba2%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637297495649621566&sdata=pbDwf%2BAG1UZ5bLZeep7VeGVQMnlQhX0TKG1d6Ok8GfQ%3D&reserved=0
> >
> > But there's a few reasons why that's not an option:
> >
> > - It's not happening in upstream, since it got reverted due to too
> > many false positives:
> >
> > 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
> >
> > This code (CONFIG_LOCKDEP_CROSSRELEASE=y and CONFIG_LOCKDEP_COMPLETIONS=y),
> > while it found a number of old bugs initially, was also causing too many
> > false positives that caused people to disable lockdep - which is arguably
> > a worse overall outcome.
> >
> > - cross-release uses the complete() call to annotate the end of
> > critical sections, for dma_fence that would be dma_fence_signal().
> > But we do not want all dma_fence_signal() calls to be treated as
> > critical, since many are opportunistic cleanup of gpu requests. If
> > these get stuck there's still the main completion interrupt and
> > workers who can unblock everyone. Automatically annotating all
> > dma_fence_signal() calls would hence cause false positives.
> >
> > - cross-release had some educated guesses for when a critical section
> > starts, like fresh syscall or fresh work callback. This would again
> > cause false positives without explicit annotations, since for
> > dma_fence the critical sections only starts when we publish a fence.
> >
> > - Furthermore there can be cases where a thread never does a
> > dma_fence_signal, but is still critical for reaching completion of
> > fences. One example would be a scheduler kthread which picks up jobs
> > and pushes them into hardware, where the interrupt handler or
> > another completion thread calls dma_fence_signal(). But if the
> > scheduler thread hangs, then all the fences hang, hence we need to
> > manually annotate it. cross-release aimed to solve this by chaining
> > cross-release dependencies, but the dependency from scheduler thread
> > to the completion interrupt handler goes through hw where
> > cross-release code can't observe it.
> >
> > In short, without manual annotations and careful review of the start
> > and end of critical sections, cross-relese dependency tracking doesn't
> > work. We need explicit annotations.
> >
> > v2: handle soft/hardirq ctx better against write side and dont forget
> > EXPORT_SYMBOL, drivers can't use this otherwise.
> >
> > v3: Kerneldoc.
> >
> > v4: Some spelling fixes from Mika
> >
> > v5: Amend commit message to explain in detail why cross-release isn't
> > the solution.
> >
> > v6: Pull out misplaced .rst hunk.
> >
> > Cc: Felix Kuehling <Felix.Kuehling at amd.com>
> > Reviewed-by: Thomas Hellström <thomas.hellstrom at intel.com>
> > Reviewed-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Mika Kuoppala <mika.kuoppala at intel.com>
> > Cc: Thomas Hellstrom <thomas.hellstrom at intel.com>
> > Cc: linux-media at vger.kernel.org
> > Cc: linaro-mm-sig at lists.linaro.org
> > Cc: linux-rdma at vger.kernel.org
> > Cc: amd-gfx at lists.freedesktop.org
> > Cc: intel-gfx at lists.freedesktop.org
> > Cc: Chris Wilson <chris at chris-wilson.co.uk>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Christian König <christian.koenig at amd.com>
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > ---
> > Documentation/driver-api/dma-buf.rst | 6 +
> > drivers/dma-buf/dma-fence.c | 161 +++++++++++++++++++++++++++
> > include/linux/dma-fence.h | 12 ++
> > 3 files changed, 179 insertions(+)
> >
> > diff --git a/Documentation/driver-api/dma-buf.rst b/Documentation/driver-api/dma-buf.rst
> > index 7fb7b661febd..05d856131140 100644
> > --- a/Documentation/driver-api/dma-buf.rst
> > +++ b/Documentation/driver-api/dma-buf.rst
> > @@ -133,6 +133,12 @@ DMA Fences
> > .. kernel-doc:: drivers/dma-buf/dma-fence.c
> > :doc: DMA fences overview
> > +DMA Fence Signalling Annotations
> > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > +
> > +.. kernel-doc:: drivers/dma-buf/dma-fence.c
> > + :doc: fence signalling annotation
> > +
> > DMA Fences Functions Reference
> > ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c
> > index 656e9ac2d028..0005bc002529 100644
> > --- a/drivers/dma-buf/dma-fence.c
> > +++ b/drivers/dma-buf/dma-fence.c
> > @@ -110,6 +110,160 @@ u64 dma_fence_context_alloc(unsigned num)
> > }
> > EXPORT_SYMBOL(dma_fence_context_alloc);
> > +/**
> > + * DOC: fence signalling annotation
> > + *
> > + * Proving correctness of all the kernel code around &dma_fence through code
> > + * review and testing is tricky for a few reasons:
> > + *
> > + * * It is a cross-driver contract, and therefore all drivers must follow the
> > + * same rules for lock nesting order, calling contexts for various functions
> > + * and anything else significant for in-kernel interfaces. But it is also
> > + * impossible to test all drivers in a single machine, hence brute-force N vs.
> > + * N testing of all combinations is impossible. Even just limiting to the
> > + * possible combinations is infeasible.
> > + *
> > + * * There is an enormous amount of driver code involved. For render drivers
> > + * there's the tail of command submission, after fences are published,
> > + * scheduler code, interrupt and workers to process job completion,
> > + * and timeout, gpu reset and gpu hang recovery code. Plus for integration
> > + * with core mm with have &mmu_notifier, respectively &mmu_interval_notifier,
> > + * and &shrinker. For modesetting drivers there's the commit tail functions
> > + * between when fences for an atomic modeset are published, and when the
> > + * corresponding vblank completes, including any interrupt processing and
> > + * related workers. Auditing all that code, across all drivers, is not
> > + * feasible.
> > + *
> > + * * Due to how many other subsystems are involved and the locking hierarchies
> > + * this pulls in there is extremely thin wiggle-room for driver-specific
> > + * differences. &dma_fence interacts with almost all of the core memory
> > + * handling through page fault handlers via &dma_resv, dma_resv_lock() and
> > + * dma_resv_unlock(). On the other side it also interacts through all
> > + * allocation sites through &mmu_notifier and &shrinker.
> > + *
> > + * Furthermore lockdep does not handle cross-release dependencies, which means
> > + * any deadlocks between dma_fence_wait() and dma_fence_signal() can't be caught
> > + * at runtime with some quick testing. The simplest example is one thread
> > + * waiting on a &dma_fence while holding a lock::
> > + *
> > + * lock(A);
> > + * dma_fence_wait(B);
> > + * unlock(A);
> > + *
> > + * while the other thread is stuck trying to acquire the same lock, which
> > + * prevents it from signalling the fence the previous thread is stuck waiting
> > + * on::
> > + *
> > + * lock(A);
> > + * unlock(A);
> > + * dma_fence_signal(B);
> > + *
> > + * By manually annotating all code relevant to signalling a &dma_fence we can
> > + * teach lockdep about these dependencies, which also helps with the validation
> > + * headache since now lockdep can check all the rules for us::
> > + *
> > + * cookie = dma_fence_begin_signalling();
> > + * lock(A);
> > + * unlock(A);
> > + * dma_fence_signal(B);
> > + * dma_fence_end_signalling(cookie);
> > + *
> > + * For using dma_fence_begin_signalling() and dma_fence_end_signalling() to
> > + * annotate critical sections the following rules need to be observed:
> > + *
> > + * * All code necessary to complete a &dma_fence must be annotated, from the
> > + * point where a fence is accessible to other threads, to the point where
> > + * dma_fence_signal() is called. Un-annotated code can contain deadlock issues,
> > + * and due to the very strict rules and many corner cases it is infeasible to
> > + * catch these just with review or normal stress testing.
> > + *
> > + * * &struct dma_resv deserves a special note, since the readers are only
> > + * protected by rcu. This means the signalling critical section starts as soon
> > + * as the new fences are installed, even before dma_resv_unlock() is called.
> > + *
> > + * * The only exception are fast paths and opportunistic signalling code, which
> > + * calls dma_fence_signal() purely as an optimization, but is not required to
> > + * guarantee completion of a &dma_fence. The usual example is a wait IOCTL
> > + * which calls dma_fence_signal(), while the mandatory completion path goes
> > + * through a hardware interrupt and possible job completion worker.
> > + *
> > + * * To aid composability of code, the annotations can be freely nested, as long
> > + * as the overall locking hierarchy is consistent. The annotations also work
> > + * both in interrupt and process context. Due to implementation details this
> > + * requires that callers pass an opaque cookie from
> > + * dma_fence_begin_signalling() to dma_fence_end_signalling().
> > + *
> > + * * Validation against the cross driver contract is implemented by priming
> > + * lockdep with the relevant hierarchy at boot-up. This means even just
> > + * testing with a single device is enough to validate a driver, at least as
> > + * far as deadlocks with dma_fence_wait() against dma_fence_signal() are
> > + * concerned.
> > + */
> > +#ifdef CONFIG_LOCKDEP
> > +struct lockdep_map dma_fence_lockdep_map = {
> > + .name = "dma_fence_map"
> > +};
> > +
> > +/**
> > + * dma_fence_begin_signalling - begin a critical DMA fence signalling section
> > + *
> > + * Drivers should use this to annotate the beginning of any code section
> > + * required to eventually complete &dma_fence by calling dma_fence_signal().
> > + *
> > + * The end of these critical sections are annotated with
> > + * dma_fence_end_signalling().
> > + *
> > + * Returns:
> > + *
> > + * Opaque cookie needed by the implementation, which needs to be passed to
> > + * dma_fence_end_signalling().
> > + */
> > +bool dma_fence_begin_signalling(void)
> > +{
> > + /* explicitly nesting ... */
> > + if (lock_is_held_type(&dma_fence_lockdep_map, 1))
> > + return true;
> > +
> > + /* rely on might_sleep check for soft/hardirq locks */
> > + if (in_atomic())
> > + return true;
> > +
> > + /* ... and non-recursive readlock */
> > + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _RET_IP_);
> > +
> > + return false;
> > +}
> > +EXPORT_SYMBOL(dma_fence_begin_signalling);
> > +
> > +/**
> > + * dma_fence_end_signalling - end a critical DMA fence signalling section
> > + *
> > + * Closes a critical section annotation opened by dma_fence_begin_signalling().
> > + */
> > +void dma_fence_end_signalling(bool cookie)
> > +{
> > + if (cookie)
> > + return;
> > +
> > + lock_release(&dma_fence_lockdep_map, _RET_IP_);
> > +}
> > +EXPORT_SYMBOL(dma_fence_end_signalling);
> > +
> > +void __dma_fence_might_wait(void)
> > +{
> > + bool tmp;
> > +
> > + tmp = lock_is_held_type(&dma_fence_lockdep_map, 1);
> > + if (tmp)
> > + lock_release(&dma_fence_lockdep_map, _THIS_IP_);
> > + lock_map_acquire(&dma_fence_lockdep_map);
> > + lock_map_release(&dma_fence_lockdep_map);
> > + if (tmp)
> > + lock_acquire(&dma_fence_lockdep_map, 0, 0, 1, 1, NULL, _THIS_IP_);
> > +}
> > +#endif
> > +
> > +
> > /**
> > * dma_fence_signal_locked - signal completion of a fence
> > * @fence: the fence to signal
> > @@ -170,14 +324,19 @@ int dma_fence_signal(struct dma_fence *fence)
> > {
> > unsigned long flags;
> > int ret;
> > + bool tmp;
> > if (!fence)
> > return -EINVAL;
> > + tmp = dma_fence_begin_signalling();
> > +
> > spin_lock_irqsave(fence->lock, flags);
> > ret = dma_fence_signal_locked(fence);
> > spin_unlock_irqrestore(fence->lock, flags);
> > + dma_fence_end_signalling(tmp);
> > +
> > return ret;
> > }
> > EXPORT_SYMBOL(dma_fence_signal);
> > @@ -210,6 +369,8 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout)
> > might_sleep();
> > + __dma_fence_might_wait();
> > +
> > trace_dma_fence_wait_start(fence);
> > if (fence->ops->wait)
> > ret = fence->ops->wait(fence, intr, timeout);
> > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > index 3347c54f3a87..3f288f7db2ef 100644
> > --- a/include/linux/dma-fence.h
> > +++ b/include/linux/dma-fence.h
> > @@ -357,6 +357,18 @@ dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)
> > } while (1);
> > }
> > +#ifdef CONFIG_LOCKDEP
> > +bool dma_fence_begin_signalling(void);
> > +void dma_fence_end_signalling(bool cookie);
> > +#else
> > +static inline bool dma_fence_begin_signalling(void)
> > +{
> > + return true;
> > +}
> > +static inline void dma_fence_end_signalling(bool cookie) {}
> > +static inline void __dma_fence_might_wait(void) {}
> > +#endif
> > +
> > int dma_fence_signal(struct dma_fence *fence);
> > int dma_fence_signal_locked(struct dma_fence *fence);
> > signed long dma_fence_default_wait(struct dma_fence *fence,
>
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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