Messages in this thread Patch in this message | | | Date | Wed, 2 Apr 2008 21:20:33 +0200 (CEST) | From | Mikulas Patocka <> | Subject | [PATCH]: Fix SMP-reordering race in mark_buffer_dirty |
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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);
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