4.4-stable review patch. If anyone has any objections, please let me know.
------------------ From: Jianyu Zhan <nasa4...@gmail.com> commit 29b75eb2d56a714190a93d7be4525e617591077a upstream. 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 <nasa4...@gmail.com> Acked-by: Christian Borntraeger <borntrae...@de.ibm.com> Acked-by: Darren Hart <dvh...@linux.intel.com> Cc: d...@stgolabs.net Cc: pet...@infradead.org Cc: li...@rasmusvillemoes.dk Cc: a...@linux-foundation.org Cc: fengguang...@intel.com Cc: bige...@linutronix.de Link: http://lkml.kernel.org/r/1457314344-5685-1-git-send-email-nasa4...@gmail.com Signed-off-by: Thomas Gleixner <t...@linutronix.de> Signed-off-by: Davidlohr Bueso <dbu...@suse.de> Signed-off-by: Greg Kroah-Hartman <gre...@linuxfoundation.org> --- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-) --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1939,8 +1939,12 @@ 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); /*