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SubjectRe: fsync fixes for 2.4


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?



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