Re: fsync fixes for 2.4

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jul 11, 2002 at 05:21:24PM -0300, Marcelo Tosatti wrote:
> 
> 
> On Wed, 10 Jul 2002, Andrea Arcangeli wrote:
> 
> > At polyserve they found a severe problem with fsync in 2.4.
> >
> > In short the write_buffer (ll_rw_block of mainline) is a noop if old I/O
> > is in flight. You know the buffer can be made dirty while I/O is in
> > flight, and in such case fsync would return without flushing the dirty
> > buffers at all. Their proposed fix is strightforward, just a
> > wait_on_buffer before the ll_rw_block will guarantee somebody marked the
> > bh locked _after_ we wrote to it.
> 
> >From what I can see the problem goes like:
> 
> 
> thread1				thread2
> 				writepage(page) (marks the buffers clean, page is
> 				locked for IO)
> 
> mark_buffer_dirty()
> 
> fsync()
> 
> fsync_buffers_list() finds
> the dirtied buffer, but since
> its locked ll_rw_block() returns
> without queueing the data.
> 
> fsync_buffers_list() waits on the writepage()'s
> write to return but not on latest data write.
> 
> 
> Is that what you mean or I'm misunderstanding something?

yes, that's it.

Andrea