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authorLinus Torvalds <torvalds@linux-foundation.org>2012-05-04 14:46:02 -0700
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2012-05-12 09:32:05 -0700
commit15e8eae4f13da852c0ed0a8b85af5ba62c46a56b (patch)
treeaeddbe4c53504bf003e540b5f098e3318fc4ab1f
parent5a3f30217f703a1e2124766bc2c5e4e4c8b57813 (diff)
Fix __read_seqcount_begin() to use ACCESS_ONCE for sequence value read
commit 2f624278626677bfaf73fef97f86b37981621f5c upstream. We really need to use a ACCESS_ONCE() on the sequence value read in __read_seqcount_begin(), because otherwise the compiler might end up reloading the value in between the test and the return of it. As a result, it might end up returning an odd value (which means that a write is in progress). If the reader is then fast enough that that odd value is still the current one when the read_seqcount_retry() is done, we might end up with a "successful" read sequence, even despite the concurrent write being active. In practice this probably never really happens - there just isn't anything else going on around the read of the sequence count, and the common case is that we end up having a read barrier immediately afterwards. So the code sequence in which gcc might decide to reaload from memory is small, and there's no reason to believe it would ever actually do the reload. But if the compiler ever were to decide to do so, it would be incredibly annoying to debug. Let's just make sure. Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--include/linux/seqlock.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index c6db9fb33c44..bb1fac5b8ee8 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -141,7 +141,7 @@ static inline unsigned __read_seqcount_begin(const seqcount_t *s)
unsigned ret;
repeat:
- ret = s->sequence;
+ ret = ACCESS_ONCE(s->sequence);
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;