Am 13.09.2019 um 11:28 hat Sergio Lopez geschrieben: > > Kevin Wolf <[email protected]> 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 <[email protected]> > >> > --- > >> > 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
signature.asc
Description: PGP signature
