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| author | Jianyu Zhan <nasa4836@gmail.com> | 2016-03-07 09:32:24 +0800 |
|---|---|---|
| committer | Thomas Gleixner <tglx@linutronix.de> | 2016-03-08 17:04:02 +0100 |
| commit | 29b75eb2d56a714190a93d7be4525e617591077a (patch) | |
| tree | 07f13028b74665bf757523864dc2478426974c54 /kernel/futex.c | |
| parent | 9e4e7554e755aaad8df0e26268787438735b4b76 (diff) | |
| download | lwn-29b75eb2d56a714190a93d7be4525e617591077a.tar.gz lwn-29b75eb2d56a714190a93d7be4525e617591077a.zip | |
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 <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>
Diffstat (limited to 'kernel/futex.c')
| -rw-r--r-- | kernel/futex.c | 8 |
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. */ 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); /* |