[PATCH 2/5] kernel.h: Add non_block_start/end()

[Daniel Vetter]

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.

v2:
- 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 :-/

Cc: Jason Gunthorpe <jgg@xxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: "Christian KÃnig" <christian.koenig@xxxxxxx>
Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
Cc: "JÃrÃme Glisse" <jglisse@xxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Cc: Wei Wang <wvw@xxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Jann Horn <jannh@xxxxxxxxxx>
Cc: Feng Tang <feng.tang@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Acked-by: Christian KÃnig <christian.koenig@xxxxxxx> (v1)
Signed-off-by: Daniel Vetter <daniel.vetter@xxxxxxxxx>
---
 include/linux/kernel.h | 10 +++++++++-
 include/linux/sched.h  |  4 ++++
 kernel/sched/core.c    | 19 ++++++++++++++-----
 3 files changed, 27 insertions(+), 6 deletions(-)

diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 4fa360a13c1e..915fd9888afb 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 +235,10 @@ 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)
+# define non_block_start() \
+	do { current->non_block_count++; } while (0)
+# define non_block_end() \
+	do { WARN_ON(current->non_block_count-- == 0); } while (0)
 #else
   static inline void ___might_sleep(const char *file, int line,
 				   int preempt_offset) { }
@@ -241,6 +247,8 @@ 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)
 #endif
 
 #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 9f51932bd543..c5630f3dca1f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -974,6 +974,10 @@ struct task_struct {
 	struct mutex_waiter		*blocked_on;
 #endif
 
+#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+	int				non_block_count;
+#endif
+
 #ifdef CONFIG_TRACE_IRQFLAGS
 	unsigned int			irq_events;
 	unsigned long			hardirq_enable_ip;
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2b037f195473..57245770d6cc 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3700,13 +3700,22 @@ 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)
 {
 #ifdef CONFIG_SCHED_STACK_END_CHECK
 	if (task_stack_end_corrupted(prev))
 		panic("corrupted stack end detected inside scheduler\n");
 #endif
 
+#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
+	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();
+		add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
+	}
+#endif
+
 	if (unlikely(in_atomic_preempt_off())) {
 		__schedule_bug(prev);
 		preempt_count_set(PREEMPT_DISABLED);
@@ -3813,7 +3822,7 @@ 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))
 		hrtick_clear(rq);
@@ -6570,7 +6579,7 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 	rcu_sleep_check();
 
 	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 ||
 	    oops_in_progress)
 		return;
@@ -6586,8 +6595,8 @@ void ___might_sleep(const char *file, int line, int preempt_offset)
 		"BUG: sleeping function called from invalid context at %s:%d\n",
 			file, line);
 	printk(KERN_ERR
-		"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))
-- 
2.22.0