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Re: [Qemu-devel] [RFC for-2.7] block: keep AioContext pointer in BlockBa

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [RFC for-2.7] block: keep AioContext pointer in BlockBackend
Date: Tue, 10 May 2016 10:33:33 +0100
User-agent: Mutt/1.6.0 (2016-04-01)

On Mon, May 09, 2016 at 06:48:12PM +0200, Paolo Bonzini wrote:
> On 11/04/2016 16:14, Stefan Hajnoczi wrote:
> > 1. BlockBackend falls back to the QEMU main loop AioContext when there
> >    is no root BlockDriverState.  This means the drive loses its
> >    AioContext during media change and would break dataplane.
> > 
> > 2. BlockBackend state used from multiple threads has no lock.  Race
> >    conditions will creep in as functionality is moved from
> >    BlockDriverState to BlockBackend due to the absense of a lock.  The
> >    monitor cannot access BlockBackend state safely while an IOThread is
> >    also accessing the state.
> > 
> > Both issue #1 and #2 are mostly theoretical at the moment.  I haven't
> > figured out a way to trigger #1 with virtio-blk (does not support media
> > change) or virtio-scsi (blocks the eject operation).  #2 may be possible
> > with block accounting statistics in BlockBackend but I'm not aware of a
> > crash that can be triggered.
> I'm not sure I agree with #2, as we should move away for the
> super-coarse AioContext lock (it is heavyweight and it doesn't play well
> with multiqueue).  Atomics can be used for statistics, while the
> time-based accounting probably should be made optional because it's
> heavy-weight and hard to scale.
> But I do agree with #1, so the idea seems good.

I agree that lightweight options are nicer.  Should I resend without #2
in the commit description?


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