Kevin Wolf <kw...@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 <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.
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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