Gitweb: http://git.kernel.org/git/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=72ed3d035855841ad611ee48b20909e9619d4a79 Commit: 72ed3d035855841ad611ee48b20909e9619d4a79 Parent: c742b53114f8d1535608dafb6a5690103a0748b5 Author: Nick Piggin <[EMAIL PROTECTED]> AuthorDate: Sat Feb 10 01:46:22 2007 -0800 Committer: Linus Torvalds <[EMAIL PROTECTED]> CommitDate: Sun Feb 11 11:15:24 2007 -0800
[PATCH] buffer: memorder fix unlock_buffer(), like unlock_page(), must not clear the lock without ensuring that the critical section is closed. Mingming later sent the same patch, saying: We are running SDET benchmark and saw double free issue for ext3 extended attributes block, which complains the same xattr block already being freed (in ext3_xattr_release_block()). The problem could also been triggered by multiple threads loop untar/rm a kernel tree. The race is caused by missing a memory barrier at unlock_buffer() before the lock bit being cleared, resulting in possible concurrent h_refcounter update. That causes a reference counter leak, then later leads to the double free that we have seen. Inside unlock_buffer(), there is a memory barrier is placed *after* the lock bit is being cleared, however, there is no memory barrier *before* the bit is cleared. On some arch the h_refcount update instruction and the clear bit instruction could be reordered, thus leave the critical section re-entered. The race is like this: For example, if the h_refcount is initialized as 1, cpu 0: cpu1 -------------------------------------- ----------------------------------- lock_buffer() /* test_and_set_bit */ clear_buffer_locked(bh); lock_buffer() /* test_and_set_bit */ h_refcount = h_refcount+1; /* = 2*/ h_refcount = h_refcount + 1; /*= 2 */ clear_buffer_locked(bh); .... ...... We lost a h_refcount here. We need a memory barrier before the buffer head lock bit being cleared to force the order of the two writes. Please apply. Signed-off-by: Nick Piggin <[EMAIL PROTECTED]> Signed-off-by: Mingming Cao <[EMAIL PROTECTED]> Signed-off-by: Andrew Morton <[EMAIL PROTECTED]> Signed-off-by: Linus Torvalds <[EMAIL PROTECTED]> --- fs/buffer.c | 1 + 1 files changed, 1 insertions(+), 0 deletions(-) diff --git a/fs/buffer.c b/fs/buffer.c index 763c5b5..7ff6e93 100644 --- a/fs/buffer.c +++ b/fs/buffer.c @@ -78,6 +78,7 @@ EXPORT_SYMBOL(__lock_buffer); void fastcall unlock_buffer(struct buffer_head *bh) { + smp_mb__before_clear_bit(); clear_buffer_locked(bh); smp_mb__after_clear_bit(); wake_up_bit(&bh->b_state, BH_Lock); - To unsubscribe from this list: send the line "unsubscribe git-commits-head" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html