Re: [Qemu-devel] [Qemu-block] [PATCH] block: avoid recursive AioContext acquire in bdrv_inactivate_all()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Dec 06, 2017 at 07:40:28PM +0100, Kevin Wolf wrote:
> Am 06.12.2017 um 18:54 hat Stefan Hajnoczi geschrieben:
> > From: Paolo Bonzini <pbonzini@redhat.com>
> > 
> > BDRV_POLL_WHILE() does not support recursive AioContext locking.  It
> > only releases the AioContext lock once regardless of how many times the
> > caller has acquired it.  This results in a hang since the IOThread does
> > not make progress while the AioContext is still locked.
> > 
> > The following steps trigger the hang:
> > 
> >   $ qemu-system-x86_64 -M accel=kvm -m 1G -cpu host \
> >                        -object iothread,id=iothread0 \
> >                        -device virtio-scsi-pci,iothread=iothread0 \
> >                        -drive if=none,id=drive0,file=test.img,format=raw \
> >                        -device scsi-hd,drive=drive0 \
> >                        -drive if=none,id=drive1,file=test.img,format=raw \
> >                        -device scsi-hd,drive=drive1
> >   $ qemu-system-x86_64 ...same options... \
> >                        -incoming tcp::1234
> >   (qemu) migrate tcp:127.0.0.1:1234
> >   ...hang...
> 
> Please turn this into a test case.
> 
> We should probably also update docs/devel/multiple-iothreads.txt.
> Currently it says:
> 
>     aio_context_acquire()/aio_context_release() calls may be nested.
>     This means you can call them if you're not sure whether #2 applies.
> 
> While technically that's still correct as far as the lock is concerned,
> the limitations of BDRV_POLL_WHILE() mean that in practice this is not a
> viable option any more at least in the context of the block layer.

Good point, will fix both things in v2.

Stefan

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