[PATCH 3/5] kernel.h: Add non_block_start/end()

Daniel Vetter daniel.vetter at ffwll.ch
Mon Aug 26 20:14:23 UTC 2019

In some special cases we must not block, but there's not a
spinlock, preempt-off, irqs-off or similar critical section already
that arms the might_sleep() debug checks. Add a non_block_start/end()
pair to annotate these.

This will be used in the oom paths of mmu-notifiers, where blocking is
not allowed to make sure there's forward progress. Quoting Michal:

"The notifier is called from quite a restricted context - oom_reaper -
which shouldn't depend on any locks or sleepable conditionals. The code
should be swift as well but we mostly do care about it to make a forward
progress. Checking for sleepable context is the best thing we could come
up with that would describe these demands at least partially."

Peter also asked whether we want to catch spinlocks on top, but Michal
said those are less of a problem because spinlocks can't have an
indirect dependency upon the page allocator and hence close the loop
with the oom reaper.

Suggested by Michal Hocko.

- Improve commit message (Michal)
- Also check in schedule, not just might_sleep (Peter)

v3: It works better when I actually squash in the fixup I had lying
around :-/

v4: Pick the suggestion from Andrew Morton to give non_block_start/end
some good kerneldoc comments. I added that other blocking calls like
wait_event pose similar issues, since that's the other example we

Cc: Jason Gunthorpe <jgg at ziepe.ca>
Cc: Peter Zijlstra <peterz at infradead.org>
Cc: Ingo Molnar <mingo at redhat.com>
Cc: Andrew Morton <akpm at linux-foundation.org>
Cc: Michal Hocko <mhocko at suse.com>
Cc: David Rientjes <rientjes at google.com>
Cc: "Christian König" <christian.koenig at amd.com>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
Cc: "Jérôme Glisse" <jglisse at redhat.com>
Cc: linux-mm at kvack.org
Cc: Masahiro Yamada <yamada.masahiro at socionext.com>
Cc: Wei Wang <wvw at google.com>
Cc: Andy Shevchenko <andriy.shevchenko at linux.intel.com>
Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Jann Horn <jannh at google.com>
Cc: Feng Tang <feng.tang at intel.com>
Cc: Kees Cook <keescook at chromium.org>
Cc: Randy Dunlap <rdunlap at infradead.org>
Cc: linux-kernel at vger.kernel.org
Acked-by: Christian König <christian.koenig at amd.com> (v1)
Acked-by: Peter Zijlstra (Intel) <peterz at infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
 include/linux/kernel.h | 25 ++++++++++++++++++++++++-
 include/linux/sched.h  |  4 ++++
 kernel/sched/core.c    | 19 ++++++++++++++-----
 3 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 4fa360a13c1e..82f84cfe372f 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -217,7 +217,9 @@ extern void __cant_sleep(const char *file, int line, int preempt_offset);
  * might_sleep - annotation for functions that can sleep
  * this macro will print a stack trace if it is executed in an atomic
- * context (spinlock, irq-handler, ...).
+ * context (spinlock, irq-handler, ...). Additional sections where blocking is
+ * not allowed can be annotated with non_block_start() and non_block_end()
+ * pairs.
  * This is a useful debugging help to be able to catch problems early and not
  * be bitten later when the calling function happens to sleep when it is not
@@ -233,6 +235,25 @@ extern void __cant_sleep(const char *file, int line, int preempt_offset);
 # define cant_sleep() \
 	do { __cant_sleep(__FILE__, __LINE__, 0); } while (0)
 # define sched_annotate_sleep()	(current->task_state_change = 0)
+ * non_block_start - annotate the start of section where sleeping is prohibited
+ *
+ * This is on behalf of the oom reaper, specifically when it is calling the mmu
+ * notifiers. The problem is that if the notifier were to block on, for example,
+ * mutex_lock() and if the process which holds that mutex were to perform a
+ * sleeping memory allocation, the oom reaper is now blocked on completion of
+ * that memory allocation. Other blocking calls like wait_event() pose similar
+ * issues.
+ */
+# define non_block_start() \
+	do { current->non_block_count++; } while (0)
+ * non_block_end - annotate the end of section where sleeping is prohibited
+ *
+ * Closes a section opened by non_block_start().
+ */
+# define non_block_end() \
+	do { WARN_ON(current->non_block_count-- == 0); } while (0)
   static inline void ___might_sleep(const char *file, int line,
 				   int preempt_offset) { }
@@ -241,6 +262,8 @@ extern void __cant_sleep(const char *file, int line, int preempt_offset);
 # define might_sleep() do { might_resched(); } while (0)
 # define cant_sleep() do { } while (0)
 # define sched_annotate_sleep() do { } while (0)
+# define non_block_start() do { } while (0)
+# define non_block_end() do { } while (0)
 #define might_sleep_if(cond) do { if (cond) might_sleep(); } while (0)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b6ec130dff9b..e8bb965f5019 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -980,6 +980,10 @@ struct task_struct {
 	struct mutex_waiter		*blocked_on;
+	int				non_block_count;
 	unsigned int			irq_events;
 	unsigned long			hardirq_enable_ip;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 45dceec209f4..0d01c7994a9a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3752,13 +3752,22 @@ static noinline void __schedule_bug(struct task_struct *prev)
  * Various schedule()-time debugging checks and statistics:
-static inline void schedule_debug(struct task_struct *prev)
+static inline void schedule_debug(struct task_struct *prev, bool preempt)
 	if (task_stack_end_corrupted(prev))
 		panic("corrupted stack end detected inside scheduler\n");
+	if (!preempt && prev->state && prev->non_block_count) {
+		printk(KERN_ERR "BUG: scheduling in a non-blocking section: %s/%d/%i\n",
+			prev->comm, prev->pid, prev->non_block_count);
+		dump_stack();
+	}
 	if (unlikely(in_atomic_preempt_off())) {
@@ -3870,7 +3879,7 @@ static void __sched notrace __schedule(bool preempt)
 	rq = cpu_rq(cpu);
 	prev = rq->curr;
-	schedule_debug(prev);
+	schedule_debug(prev, preempt);
 	if (sched_feat(HRTICK))
@@ -6641,7 +6650,7 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 	if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
-	     !is_idle_task(current)) ||
+	     !is_idle_task(current) && !current->non_block_count) ||
 	    system_state == SYSTEM_BOOTING || system_state > SYSTEM_RUNNING ||
@@ -6657,8 +6666,8 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 		"BUG: sleeping function called from invalid context at %s:%d\n",
 			file, line);
-		"in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n",
-			in_atomic(), irqs_disabled(),
+		"in_atomic(): %d, irqs_disabled(): %d, non_block: %d, pid: %d, name: %s\n",
+			in_atomic(), irqs_disabled(), current->non_block_count,
 			current->pid, current->comm);
 	if (task_stack_end_corrupted(current))

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