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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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