Re: fsync fixes for 2.4

[Marcelo Tosatti]

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.

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?