[PATCH] locking/mutex: Add debug code to help catching violation of mutex lifetime rule

[Waiman Long <longman@redhat.com>]

Callers of mutex_lock/unlock() must ensure the validity of the memory
objects holding mutexes until after the return of all outstanding
and upcoming mutex_unlock() calls. Add a cond_resched() call in
__mutex_unlock_slowpath() to help KASAN catch a violation of this
rule. This will enforce a context switch if another runnable task is
present and the CPU is not in an atomic context.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/mutex.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index a39ecccbd106..dd107fb9dad7 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -931,6 +931,17 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
 		}
 	}
 
+#if defined(CONFIG_KASAN) && defined(CONFIG_DEBUG_MUTEXES)
+	/*
+	 * Mutex users must ensure that the memory object holding the mutex
+	 * remains valid until after the return of all the outstanding and
+	 * upcoming mutex_unlock() calls. Enforce a context switch here if
+	 * another runnable task is present and this CPU is not in an atomic
+	 * context to increase the chance that KASAN can catch a violation of
+	 * this rule.
+	 */
+	cond_resched();
+#endif
 	raw_spin_lock_irqsave(&lock->wait_lock, flags);
 	debug_mutex_unlock(lock);
 	if (!list_empty(&lock->wait_list)) {
-- 
2.50.0