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 <[email protected]> > Cc: [email protected] > > > Reported-by: Qianqian Zhu <[email protected]> > > Suggested-by: Fam Zheng <[email protected]> > > Signed-off-by: Stefan Hajnoczi <[email protected]> > > --- > > 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); > > } > > > > /** > >
