Re: [Qemu-devel] [PATCH] aio: add missing aio_notify() to aio_enable_external()

[Fam Zheng <famz@redhat.com>]

On Thu, 05/04 12:36, Paolo Bonzini wrote:
> 
> 
> On 04/05/2017 12:23, Stefan Hajnoczi wrote:
> > The main loop uses aio_disable_external()/aio_enable_external() to
> > temporarily disable processing of external AioContext clients like
> > device emulation.
> > 
> > This allows monitor commands to quiesce I/O and prevent the guest from
> > submitting new requests while a monitor command is in progress.
> > 
> > The aio_enable_external() API is currently broken when an IOThread is in
> > aio_poll() waiting for fd activity when the main loop re-enables
> > external clients.  Incrementing ctx->external_disable_cnt does not wake
> > the IOThread from ppoll(2) so fd processing remains suspended and leads
> > to unresponsive emulated devices.
> > 
> > This patch adds an aio_notify() call to aio_enable_external() so the
> > IOThread is kicked out of ppoll(2) and will re-arm the file descriptors.
> > 
> > The bug can be reproduced as follows:
> > 
> >   $ qemu -M accel=kvm -m 1024 \
> >          -object iothread,id=iothread0 \
> >          -device virtio-scsi-pci,iothread=iothread0,id=virtio-scsi-pci0 \
> >          -drive if=none,id=drive0,aio=native,cache=none,format=raw,file=test.img \
> >          -device scsi-hd,id=scsi-hd0,drive=drive0 \
> >          -qmp tcp::5555,server,nowait
> > 
> >   $ scripts/qmp/qmp-shell localhost:5555
> >   (qemu) blockdev-snapshot-sync device=drive0 snapshot-file=sn1.qcow2
> >          mode=absolute-paths format=qcow2
> > 
> > After blockdev-snapshot-sync completes the SCSI disk will be
> > unresponsive.  This leads to request timeouts inside the guest.
> 
> I agree this is the minimal fix and is the right thing to do.  The
> bdrv_drained_begin/end device callbacks would also make it possible to
> remove disable/enable external altogether, but that's more invasive.
> 
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Cc: qemu-stable@nongnu.org
> 
> > Reported-by: Qianqian Zhu <qizhu@redhat.com>
> > Suggested-by: Fam Zheng <famz@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  include/block/aio.h | 1 +
> >  1 file changed, 1 insertion(+)
> > 
> > diff --git a/include/block/aio.h b/include/block/aio.h
> > index 406e323..5294b04 100644
> > --- a/include/block/aio.h
> > +++ b/include/block/aio.h
> > @@ -456,6 +456,7 @@ static inline void aio_enable_external(AioContext *ctx)
> >  {
> >      assert(ctx->external_disable_cnt > 0);
> >      atomic_dec(&ctx->external_disable_cnt);

This can be changed to atomic_fetch_dec and only call aio_notify if it returned
1, which is cleaner.

> > +    aio_notify(ctx);
> >  }
> >  
> >  /**
> >