Re: [Qemu-devel] [PATCH] vl: pause vcpus before stopping iothreads

[Kevin Wolf <kwolf@redhat.com>]

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Am 31.01.2018 um 14:56 hat Stefan Hajnoczi geschrieben:
> On Tue, Jan 30, 2018 at 05:54:56PM +0100, Kevin Wolf wrote:
> > Am 30.01.2018 um 16:38 hat Stefan Hajnoczi geschrieben:
> > > Commit dce8921b2baaf95974af8176406881872067adfa ("iothread: Stop threads
> > > before main() quits") introduced iothread_stop_all() to avoid the
> > > following virtio-scsi assertion failure:
> > > 
> > >   assert(blk_get_aio_context(d->conf.blk) == s->ctx);
> > > 
> > > Back then the assertion failed because when bdrv_close_all() made
> > > d->conf.blk NULL, blk_get_aio_context() returned the global AioContext
> > > instead of s->ctx.
> > > 
> > > The same assertion can still fail today when vcpus submit new I/O
> > > requests after iothread_stop_all() has moved the BDS to the global
> > > AioContext.
> > > 
> > > This patch hardens the iothread_stop_all() approach by pausing vcpus
> > > before calling iothread_stop_all().
> > > 
> > > Note that the assertion failure is a race condition.  It is not possible
> > > to reproduce it reliably.
> > > 
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > Does pausing the vcpus actually make sure that the iothread isn't active
> > any more, or do we still have a small window where the vcpu is already
> > stopped, but the iothread is still processing requests?
> > 
> > Essentially, I think the bdrv_set_aio_context() in iothread_stop_all()
> > does either not have any effect, or if it does have an effect, it's
> > wrong. You can't just force an in-use BDS into a different AioContext
> > when the user that set the AioContext is still there.
> > 
> > At the very least, do we need a blk_drain_all() before stopping the
> > iothreads?
> 
> bdrv_set_aio_context() contains aio_disable_external() +
> bdrv_parent_drained_begin() + bdrv_drain(bs).  This should complete all
> requests, even those sitting in a descriptor ring that hasn't been
> processed yet.

Ah, yes. Not very obvious, so I wouldn't mind a comment, but you can
have my R-b either way then:

Reviewed-by: Kevin Wolf <kwolf@redhat.com>

> > It would still just be a hack, the proper way seens to be
> > getting the virtio device out of dataplane mode so that the iothread is
> > actually unused and doesn't just happen to not process something at the
> > moment.
> 
> Agreed, the existing approach is a hack.  I'm not keen on implementing
> a proper device<->IOThread detach operation because vl.c:main() seems to
> be the only place that needs it - and it can get away with just
> quiescing requests and the IOThread instead.

As long as we don't want to switch devices between iothreads at runtime
(and create/delete iothreads over QMP), we probably won't really need
it, yes.

Kevin

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