From e274795ea7b7caa0fd74ef651594382a69e2a951 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 11 Jan 2017 14:17:48 +0100 Subject: locking/mutex: Fix mutex handoff While reviewing the ww_mutex patches, I noticed that it was still possible to (incorrectly) succeed for (incorrect) code like: mutex_lock(&a); mutex_lock(&a); This was possible if the second mutex_lock() would block (as expected) but then receive a spurious wakeup. At that point it would find itself at the front of the queue, request a handoff and instantly claim ownership and continue, since owner would point to itself. Avoid this scenario and simplify the code by introducing a third low bit to signal handoff pickup. So once we request handoff, unlock clears the handoff bit and sets the pickup bit along with the new owner. This also removes the need for the .handoff argument to __mutex_trylock(), since that becomes superfluous with PICKUP. In order to guarantee enough low bits, ensure task_struct alignment is at least L1_CACHE_BYTES (which seems a good ideal regardless). Signed-off-by: Peter Zijlstra (Intel) Cc: Andrew Morton Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Peter Zijlstra Cc: Thomas Gleixner Fixes: 9d659ae14b54 ("locking/mutex: Add lock handoff to avoid starvation") Signed-off-by: Ingo Molnar --- include/linux/mutex.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/mutex.h') diff --git a/include/linux/mutex.h b/include/linux/mutex.h index b97870f2debd..3e1fccb47f11 100644 --- a/include/linux/mutex.h +++ b/include/linux/mutex.h @@ -65,7 +65,7 @@ struct mutex { static inline struct task_struct *__mutex_owner(struct mutex *lock) { - return (struct task_struct *)(atomic_long_read(&lock->owner) & ~0x03); + return (struct task_struct *)(atomic_long_read(&lock->owner) & ~0x07); } /* -- cgit