Re: [Qemu-devel] [PATCH] qed: fix bdrv_qed_drain

[Fam Zheng <famz@redhat.com>]

On Wed, 02/17 12:28, Paolo Bonzini wrote:
> 
> 
> On 17/02/2016 03:57, Fam Zheng wrote:
> > On Tue, 02/16 16:53, Paolo Bonzini wrote:
> >> The current implementation of bdrv_qed_drain can cause a double
> >> completion of a request.
> >>
> >> The problem is that bdrv_qed_drain calls qed_plug_allocating_write_reqs
> >> unconditionally, but this is not correct if an allocating write
> >> is queued.  In this case, qed_unplug_allocating_write_reqs will
> >> restart the allocating write and possibly cause it to complete.
> >> The aiocb however is still in use for the L2/L1 table writes,
> >> and will then be completed again as soon as the table writes
> >> are stable.
> >>
> >> The fix is to only call qed_plug_allocating_write_reqs and
> >> bdrv_aio_flush (which is the same as the timer callback---the patch
> >> makes this explicit) only if the timer was scheduled in the first
> >> place.  This fixes qemu-iotests test 011.
> >>
> >> Cc: qemu-stable@nongnu.org
> >> Cc: qemu-block@nongnu.org
> >> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> >> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> >> ---
> >>  block/qed.c | 13 +++++++------
> >>  1 file changed, 7 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/block/qed.c b/block/qed.c
> >> index 404be1e..ebba220 100644
> >> --- a/block/qed.c
> >> +++ b/block/qed.c
> >> @@ -380,12 +380,13 @@ static void bdrv_qed_drain(BlockDriverState *bs)
> >>  {
> >>      BDRVQEDState *s = bs->opaque;
> >>  
> >> -    /* Cancel timer and start doing I/O that were meant to happen as if it
> >> -     * fired, that way we get bdrv_drain() taking care of the ongoing requests
> >> -     * correctly. */
> >> -    qed_cancel_need_check_timer(s);
> >> -    qed_plug_allocating_write_reqs(s);
> >> -    bdrv_aio_flush(s->bs, qed_clear_need_check, s);
> >> +    /* Fire the timer immediately in order to start doing I/O as soon as the
> >> +     * header is flushed.
> >> +     */
> >> +    if (s->need_check_timer && timer_pending(s->need_check_timer)) {
> > 
> > We can assert(s->need_check_timer);
> 
> I've seen it NULL, but didn't check why.  This was also a source of
> segmentation faults.
> 
> >> +        qed_cancel_need_check_timer(s);
> >> +        qed_need_check_timer_cb(s);
> >> +    }
> > 
> > What if an allocating write is queued (the else branch case)? Its completion
> > will be in bdrv_drain and it could arm the need_check_timer which is wrong.
> > 
> > We need to drain the allocating_write_reqs queue before checking the timer.
> 
> You're right, but how?  That's what bdrv_drain(bs) does, it's a
> chicken-and-egg problem.

Maybe use an aio_poll loop before the if?

Fam