Re: [Qemu-devel] [PATCH 3/3] linux-aio: fix re-entrant completion processing

[Fam Zheng <famz@redhat.com>]

On Wed, 09/28 11:14, Roman Penyaev wrote:
> On Wed, Sep 28, 2016 at 5:01 AM, Fam Zheng <famz@redhat.com> wrote:
> > On Tue, 09/27 19:55, Roman Penyaev wrote:
> >> > The bug is 100% deterministic.  Just boot up a guest with -drive
> >> > format=qcow2,aio=native.
> >>
> >> It turns out to be that everything is broken.  I started all my
> >> tests with format=raw,aio=native and immediately got coroutine
> >> recursive.  That is completely weird.
> >>
> >> So, what I did is the following:
> >>
> >> 1. Took latest master (nothing works)
> >> 2. Did interactive rebase to 12c8720
> >> 12c8720 2016-06-28 | Merge remote-tracking branch
> >> 'remotes/stefanha/tags/block-pull-request' into staging [Peter
> >> Maydell]
> >>
> >> this merge request includes all your patches related to
> >> virtio-blk and MQ support.
> >>
> >> 3. Applied 0ed93d84edab. Everything works fine.
> >
> > Have you tried qcow2 at this point? raw crashes with 1a62d0accdf85 doesn't mean
> > qcow2 is fine without it.
> >
> 
> That's true.  qcow2 IO path is different, and presence of the
> patch 1a62d0accdf85 does not affect - coroutine still enters
> recursively.
> 
> But for me it is quite surprising that IO fragmentation (what
> was done in 1a62d0accdf85) rises the misbehavior on raw IO path.

Maybe the mystery with this change is your particular I/O pattern on the raw
image is change thereafter, from ioq = 1 to ioq > 1 (from the linux-aio.c's
PoV, due to fragmentation), then multiple coroutines are created for one big
request, to trigger the crash.

Fam

> 
> But of course originally issue was introduced by me.  Stefan,
> thanks for a fix.
> 
> --
> Roman