diff options
authorJianyu Zhan <>2016-03-07 09:32:24 +0800
committerThomas Gleixner <>2016-03-08 17:04:02 +0100
commit29b75eb2d56a714190a93d7be4525e617591077a (patch)
parent9e4e7554e755aaad8df0e26268787438735b4b76 (diff)
futex: Replace barrier() in unqueue_me() with READ_ONCE()
Commit e91467ecd1ef ("bug in futex unqueue_me") introduced a barrier() in unqueue_me() to prevent the compiler from rereading the lock pointer which might change after a check for NULL. Replace the barrier() with a READ_ONCE() for the following reasons: 1) READ_ONCE() is a weaker form of barrier() that affects only the specific load operation, while barrier() is a general compiler level memory barrier. READ_ONCE() was not available at the time when the barrier was added. 2) Aside of that READ_ONCE() is descriptive and self explainatory while a barrier without comment is not clear to the casual reader. No functional change. [ tglx: Massaged changelog ] Signed-off-by: Jianyu Zhan <> Acked-by: Christian Borntraeger <> Acked-by: Darren Hart <> Cc: Cc: Cc: Cc: Cc: Cc: Link: Signed-off-by: Thomas Gleixner <>
1 files changed, 6 insertions, 2 deletions
diff --git a/kernel/futex.c b/kernel/futex.c
index bae542e4b2e9..a5d2e74c89e0 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2010,8 +2010,12 @@ static int unqueue_me(struct futex_q *q)
/* In the common case we don't take the spinlock, which is nice. */
- lock_ptr = q->lock_ptr;
- barrier();
+ /*
+ * q->lock_ptr can change between this read and the following spin_lock.
+ * Use READ_ONCE to forbid the compiler from reloading q->lock_ptr and
+ * optimizing lock_ptr out of the logic below.
+ */
+ lock_ptr = READ_ONCE(q->lock_ptr);
if (lock_ptr != NULL) {