This is a note to let you know that I've just added the patch titled
Fix __read_seqcount_begin() to use ACCESS_ONCE for sequence value read
to the 3.0-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
fix-__read_seqcount_begin-to-use-access_once-for-sequence-value-read.patch
and it can be found in the queue-3.0 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <[email protected]> know about it.
>From 2f624278626677bfaf73fef97f86b37981621f5c Mon Sep 17 00:00:00 2001
From: Linus Torvalds <[email protected]>
Date: Fri, 4 May 2012 14:46:02 -0700
Subject: Fix __read_seqcount_begin() to use ACCESS_ONCE for sequence value read
From: Linus Torvalds <[email protected]>
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 <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>
---
include/linux/seqlock.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -141,7 +141,7 @@ static inline unsigned __read_seqcount_b
unsigned ret;
repeat:
- ret = s->sequence;
+ ret = ACCESS_ONCE(s->sequence);
if (unlikely(ret & 1)) {
cpu_relax();
goto repeat;
Patches currently in stable-queue which might be from
[email protected] are
queue-3.0/fix-__read_seqcount_begin-to-use-access_once-for-sequence-value-read.patch
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