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

[Nick Piggin <nickpiggin@yahoo.com.au>]

Andrew Morton wrote:
> 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)
> 

I don't think b) happens, because thread 1 has to have finished all
its writes before the page will end writeback and thread 2 can go
anywhere.

>    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?
> 

Well you really don't need to hold the page locked for that long.
block_read_full_page, nobh_prepare_write both use the same sort of
array of buffer heads logic - I think it makes sense not to touch
any buffers after submitting them all for IO...?

-- 
SUSE Labs, Novell Inc.