lkml.org 
[lkml]   [2008]   [Apr]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH]: Fix SMP-reordering race in mark_buffer_dirty
Hi

It looks like someone overoptimized mark_buffer_dirty().

mark_buffer_dirty() is
void mark_buffer_dirty(struct buffer_head *bh)
{
WARN_ON_ONCE(!buffer_uptodate(bh));
if (!buffer_dirty(bh) && !test_set_buffer_dirty(bh))
__set_page_dirty(bh->b_page, page_mapping(bh->b_page), 0);
}
That buffer_dirty() test is not atomic, it may be reordered with whatever
else.

So suppose this race

CPU1:

write to buffer data
call mark_buffer_dirty()
test for !buffer_dirty(bh)

--- there is no synchronizing operation, so inside CPU it may get
reordered to:
test for !buffer_dirty(bh)
write to buffer data

CPU2:
clear_buffer_dirty(bh);
submit_bh(WRITE, bh);

The resulting operations may end up in this order:
CPU1: test for !buffer_dirty(bh) --- sees that the bit is set
CPU2: clear_buffer_dirty(bh);
CPU2: submit_bh(WRITE, bh);
CPU1: write to buffer data

So we have a clean buffer with modified data and this modification is
going to be lost.

Mikulas


Signed-off-by: Mikulas Patocka <mikulas@artax.karlin.mff.cuni.cz>

--- linux-2.6.25-rc8/fs/buffer.c_ 2008-04-02 21:08:36.000000000 +0200
+++ linux-2.6.25-rc8/fs/buffer.c 2008-04-02 21:10:25.000000000 +0200
@@ -1180,6 +1180,12 @@
*/
void mark_buffer_dirty(struct buffer_head *bh)
{
+ /*
+ * Make sure that the test for buffer_dirty(bh) is not reordered with
+ * previous modifications to the buffer data.
+ * -- mikulas
+ */
+ smp_mb();
WARN_ON_ONCE(!buffer_uptodate(bh));
if (!buffer_dirty(bh) && !test_set_buffer_dirty(bh))
__set_page_dirty(bh->b_page, page_mapping(bh->b_page), 0);

\
 
 \ /
  Last update: 2008-04-02 21:23    [from the cache]
©2003-2011 Jasper Spaans