The patch titled
buffer: memorder fix
has been added to the -mm tree. Its filename is
buffer-memorder-fix.patch
*** Remember to use Documentation/SubmitChecklist when testing your code ***
See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find
out what to do about this
------------------------------------------------------
Subject: buffer: memorder fix
From: Nick Piggin <npiggin@xxxxxxx>
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 <npiggin@xxxxxxx>
Cc: Mingming Cao <cmm@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
---
fs/buffer.c | 1 +
1 files changed, 1 insertion(+)
diff -puN fs/buffer.c~buffer-memorder-fix fs/buffer.c
--- a/fs/buffer.c~buffer-memorder-fix
+++ a/fs/buffer.c
@@ -70,6 +70,7 @@ 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);
_
Patches currently in -mm which might be from npiggin@xxxxxxx are
git-block.patch
buffer-memorder-fix.patch
sched-avoid-div-in-rebalance_tick.patch
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