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 <s...@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. 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
