From: Jianyu Zhan nasa4836@gmail.com
This patch has been added to the 4.1 stable tree. If you have any objections, please let us know.
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[ Upstream commit 29b75eb2d56a714190a93d7be4525e617591077a ]
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 nasa4836@gmail.com Acked-by: Christian Borntraeger borntraeger@de.ibm.com Acked-by: Darren Hart dvhart@linux.intel.com Cc: dave@stgolabs.net Cc: peterz@infradead.org Cc: linux@rasmusvillemoes.dk Cc: akpm@linux-foundation.org Cc: fengguang.wu@intel.com Cc: bigeasy@linutronix.de Link: http://lkml.kernel.org/r/1457314344-5685-1-git-send-email-nasa4836@gmail.com Signed-off-by: Thomas Gleixner tglx@linutronix.de Signed-off-by: Sasha Levin alexander.levin@microsoft.com --- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index 585cf96dab32..959cc4fc6de5 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1831,8 +1831,12 @@ static int unqueue_me(struct futex_q *q)
/* In the common case we don't take the spinlock, which is nice. */ retry: - 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) { spin_lock(lock_ptr); /*