Re: fsync fixes for 2.4

[Andrea Arcangeli <andrea@suse.de>]

On Thu, Jul 11, 2002 at 09:51:29PM -0300, Marcelo Tosatti wrote:
> 
> 
> On Fri, 12 Jul 2002, Andrea Arcangeli wrote:
> 
> > 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.
> 
> So I'm just going to add wait_on_page() on fsync_buffers_list() before the
> ll_rw_block().
> 
> Nothing else, since all of the other stuff on your patch seems to be
> improvements rather than bug fixes. ACK?

agreed, for an rc2 that's certainly the best approch, thanks.

Andrea