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

[Kevin Wolf]

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 <address@hidden>
> > ---
> >  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.

The reason why it deadlocks also has nothing to do with whether we are
called from a coroutine or not. The important part is that we're running
in an iothread.

Kevin