Re: [patch] fix the 2nd buffer race properly

Andrew Morton
2005-04-27T18:00:53Z

Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
> When running
> fsstress -v -d $DIR/tmp -n 1000 -p 1000 -l 2
> on an ext2 filesystem with 1024 byte block size, on SMP i386 with 4096 byte
> page size over loopback to an image file on a tmpfs filesystem, I would
> very quickly hit
> BUG_ON(!buffer_async_write(bh));
> in fs/buffer.c:end_buffer_async_write
>
> It seems that more than one request would be submitted for a given bh
> at a time.
>
> What would happen is the following:
> 2 threads doing __mpage_writepages on the same page.
> Thread 1 - lock the page first, and enter __block_write_full_page.
> Thread 1 - (eg.) mark_buffer_async_write on the first 2 buffers.
> Thread 1 - set page writeback, unlock page.
> Thread 2 - lock page, wait on page writeback
> Thread 1 - submit_bh on the first 2 buffers.
> => both requests complete, none of the page buffers are async_write,
> end_page_writeback is called.
> Thread 2 - wakes up. enters __block_write_full_page.
> Thread 2 - mark_buffer_async_write on (eg.) the last buffer
> Thread 1 - finds the last buffer has async_write set, submit_bh on that.
> Thread 2 - submit_bh on the last buffer.
> => oops.

ah-hah. Thanks.

There are two situations:

a) Thread 2 comes in and tries to write a buffer which thread1 didn't write:

Yes, thread 1 will get confused and will try to write thread 2's buffer.

b) Thread 2 comes in and tries to write a buffer which thread 1 is
writing. (Say, the buffer was redirtied by
munmap->__set_page_dirty_buffers, which doesn't lock the page or the
buffers)

Thread 2 will fail the test_set_buffer_locked() and will redirty the page.

That's all a bit too complex. How's about this instead?

--- 25/fs/buffer.c~fix-race-in-block_write_full_page 2005-04-27 10:42:11.191956704 -0700
+++ 25-akpm/fs/buffer.c 2005-04-27 10:42:56.548061528 -0700
@@ -1837,7 +1837,6 @@ static int __block_write_full_page(struc
*/
BUG_ON(PageWriteback(page));
set_page_writeback(page);
- unlock_page(page);

do {
struct buffer_head *next = bh->b_this_page;
@@ -1848,6 +1847,7 @@ static int __block_write_full_page(struc
put_bh(bh);
bh = next;
} while (bh != head);
+ unlock_page(page);

err = 0;
done:
@@ -1901,7 +1901,6 @@ recover:
SetPageError(page);
BUG_ON(PageWriteback(page));
set_page_writeback(page);
- unlock_page(page);
do {
struct buffer_head *next = bh->b_this_page;
if (buffer_async_write(bh)) {
@@ -1912,6 +1911,7 @@ recover:
put_bh(bh);
bh = next;
} while (bh != head);
+ unlock_page(page);
goto done;
}

_

Aside: can the redirty_page_for_writepage() ever happen any more? Can a
buffer against a locked page be locked by some other actor? I guess so -
kjournald in ordered mode might be trying to write the buffer as well,
perhaps...