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? > 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); > + } > } > > static const BlockDevOps virtio_block_ops = { > -- > 2.21.0
