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 <[email protected]> > > > > 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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