Re: [Qemu-devel] [RFC PATCH] virtio-blk: schedule virtio_notify_config to run on main context

[Kevin Wolf <kwolf@redhat.com>]

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Am 13.09.2019 um 11:28 hat Sergio Lopez geschrieben:
> 
> Kevin Wolf <kwolf@redhat.com> writes:
> 
> > Am 12.09.2019 um 21:51 hat Michael S. Tsirkin geschrieben:
> >> On Thu, Sep 12, 2019 at 08:19:25PM +0200, Sergio Lopez wrote:
> >> > Another AioContext-related issue, and this is a tricky one.
> >> > 
> >> > Executing a QMP block_resize request for a virtio-blk device running
> >> > on an iothread may cause a deadlock involving the following mutexes:
> >> > 
> >> >  - main thead
> >> >   * Has acquired: qemu_mutex_global.
> >> >   * Is trying the acquire: iothread AioContext lock via
> >> >     AIO_WAIT_WHILE (after aio_poll).
> >> > 
> >> >  - iothread
> >> >   * Has acquired: AioContext lock.
> >> >   * Is trying to acquire: qemu_mutex_global (via
> >> >     virtio_notify_config->prepare_mmio_access).
> >> 
> >> Hmm is this really the only case iothread takes qemu mutex?
> >> If any such access can deadlock, don't we need a generic
> >> solution? Maybe main thread can drop qemu mutex
> >> before taking io thread AioContext lock?
> >
> > The rule is that iothreads must not take the qemu mutex. If they do
> > (like in this case), it's a bug.
> >
> > Maybe we could actually assert this in qemu_mutex_lock_iothread()?
> >
> >> > With this change, virtio_blk_resize checks if it's being called from a
> >> > coroutine context running on a non-main thread, and if that's the
> >> > case, creates a new coroutine and schedules it to be run on the main
> >> > thread.
> >> > 
> >> > This works, but means the actual operation is done
> >> > asynchronously, perhaps opening a window in which a "device_del"
> >> > operation may fit and remove the VirtIODevice before
> >> > virtio_notify_config() is executed.
> >> > 
> >> > I *think* it shouldn't be possible, as BHs will be processed before
> >> > any new QMP/monitor command, but I'm open to a different approach.
> >> > 
> >> > RHBZ: https://bugzilla.redhat.com/show_bug.cgi?id=1744955
> >> > Signed-off-by: Sergio Lopez <slp@redhat.com>
> >> > ---
> >> >  hw/block/virtio-blk.c | 25 ++++++++++++++++++++++++-
> >> >  1 file changed, 24 insertions(+), 1 deletion(-)
> >> > 
> >> > diff --git a/hw/block/virtio-blk.c b/hw/block/virtio-blk.c
> >> > index 18851601cb..c763d071f6 100644
> >> > --- a/hw/block/virtio-blk.c
> >> > +++ b/hw/block/virtio-blk.c
> >> > @@ -16,6 +16,7 @@
> >> >  #include "qemu/iov.h"
> >> >  #include "qemu/module.h"
> >> >  #include "qemu/error-report.h"
> >> > +#include "qemu/main-loop.h"
> >> >  #include "trace.h"
> >> >  #include "hw/block/block.h"
> >> >  #include "hw/qdev-properties.h"
> >> > @@ -1086,11 +1087,33 @@ static int virtio_blk_load_device(VirtIODevice *vdev, QEMUFile *f,
> >> >      return 0;
> >> >  }
> >> >  
> >> > +static void coroutine_fn virtio_resize_co_entry(void *opaque)
> >> > +{
> >> > +    VirtIODevice *vdev = opaque;
> >> > +
> >> > +    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
> >> > +    virtio_notify_config(vdev);
> >> > +    aio_wait_kick();
> >> > +}
> >> > +
> >> >  static void virtio_blk_resize(void *opaque)
> >> >  {
> >> >      VirtIODevice *vdev = VIRTIO_DEVICE(opaque);
> >> > +    Coroutine *co;
> >> >  
> >> > -    virtio_notify_config(vdev);
> >> > +    if (qemu_in_coroutine() &&
> >> > +        qemu_get_current_aio_context() != qemu_get_aio_context()) {
> >> > +        /*
> >> > +         * virtio_notify_config() needs to acquire the global mutex,
> >> > +         * so calling it from a coroutine running on a non-main context
> >> > +         * may cause a deadlock. Instead, create a new coroutine and
> >> > +         * schedule it to be run on the main thread.
> >> > +         */
> >> > +        co = qemu_coroutine_create(virtio_resize_co_entry, vdev);
> >> > +        aio_co_schedule(qemu_get_aio_context(), co);
> >> > +    } else {
> >> > +        virtio_notify_config(vdev);
> >> > +    }
> >> >  }
> >
> > Wouldn't a simple BH suffice (aio_bh_schedule_oneshot)? I don't see why
> > you need a coroutine when you never yield.
> 
> You're right, that's actually simpler, haven't thought of it.
> 
> Do you see any drawbacks or should I send a non-RFC fixed version of
> this patch?

Sending a fixed non-RFC version sounds good to me.

Kevin

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