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

[Sergio Lopez <slp@redhat.com>]

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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?

> 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


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