Messages in this thread Patch in this message | | | From | Jianyu Zhan <> | Subject | [PATCH] futex: replace bare barrier() with a READ_ONCE() | Date | Mon, 29 Feb 2016 19:41:33 +0800 |
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Commit e91467ecd1ef ("bug in futex unqueue_me") introduces a barrier() in unqueue_me(), to address a pointer aliasing problem in s390x, between q->lock_ptr and local variable lock_ptr.
Since there is a retry logic to reload q->lock_ptr into local variable lock_ptr, s390x generates code that aliases q->lock_ptr with lock_ptr, while q->lock_ptr might change between retries, which beats the alias and causes problem.
This patch replaces this bare barrier() with a READ_ONCE(), a weaker form of barrier(), which could be more informative.
And furthermore, this retry logic is effectively the same with:
while (lock_ptr = q->lock_ptr) do_something_with(lock_ptr);
and compiler is at its will to merge successive load of q->lock_ptr, which is also problematic at this scenario. READ_ONCE() can avoid this problem.
Signed-off-by: Jianyu Zhan <nasa4836@gmail.com> --- kernel/futex.c | 8 ++++++-- 1 file changed, 6 insertions(+), 2 deletions(-)
diff --git a/kernel/futex.c b/kernel/futex.c index 5d6ce64..20e8466 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1927,8 +1927,12 @@ 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 and a pointer aliasing(of lock_ptr) will cause problem, + * and also to avoid potential compiler merging of successive load of q->lock_ptr under + * this retry logic, so we use READ_ONCE() here. + */ + lock_ptr = READ_ONCE(q->lock_ptr); if (lock_ptr != NULL) { spin_lock(lock_ptr); /* -- 2.4.3
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