Re: [Qemu-devel] [PATCH] virtio-blk: dataplane: release AioContext before blk_set_aio_context

[Kevin Wolf <kwolf@redhat.com>]

Am 27.02.2019 um 17:52 hat Sergio Lopez geschrieben:
> Stopping the dataplane requires calling to blk_set_aio_context, which
> may need to wait for a running job to be completed or paused.
> 
> As stopping the dataplane is something that can be triggered from a vcpu
> thread (due to the Guest requesting to stop the device), while the job
> itself may be managed by an iothread, holding the AioContext will lead
> to a deadlock, where the first one is waiting for the job to pause or
> finish, while the second can't make any progress as it's waiting for the
> AioContext to be released:
> 
>  Thread 6 (LWP 90472)
>  #0  0x000055a6f8497aee in blk_root_drained_end
>  #1  0x000055a6f84a7926 in bdrv_parent_drained_end
>  #2  0x000055a6f84a799f in bdrv_do_drained_end
>  #3  0x000055a6f84a82ab in bdrv_drained_end
>  #4  0x000055a6f8498be8 in blk_drain
>  #5  0x000055a6f84a22cd in mirror_drain
>  #6  0x000055a6f8457708 in block_job_detach_aio_context
>  #7  0x000055a6f84538f1 in bdrv_detach_aio_context
>  #8  0x000055a6f8453a96 in bdrv_set_aio_context
>  #9  0x000055a6f84999f8 in blk_set_aio_context
>  #10 0x000055a6f81802c8 in virtio_blk_data_plane_stop
>  #11 0x000055a6f83cfc95 in virtio_bus_stop_ioeventfd
>  #12 0x000055a6f83d1f81 in virtio_pci_common_write
>  #13 0x000055a6f83d1f81 in virtio_pci_common_write
>  #14 0x000055a6f8148d62 in memory_region_write_accessor
>  #15 0x000055a6f81459c9 in access_with_adjusted_size
>  #16 0x000055a6f814a608 in memory_region_dispatch_write
>  #17 0x000055a6f80f1f98 in flatview_write_continue
>  #18 0x000055a6f80f214f in flatview_write
>  #19 0x000055a6f80f6a7b in address_space_write
>  #20 0x000055a6f80f6b15 in address_space_rw
>  #21 0x000055a6f815da08 in kvm_cpu_exec
>  #22 0x000055a6f8132fee in qemu_kvm_cpu_thread_fn
>  #23 0x000055a6f8551306 in qemu_thread_start
>  #24 0x00007f9bdf5b9dd5 in start_thread
>  #25 0x00007f9bdf2e2ead in clone
> 
>  Thread 8 (LWP 90467)
>  #0  0x00007f9bdf5c04ed in __lll_lock_wait
>  #1  0x00007f9bdf5bbde6 in _L_lock_941
>  #2  0x00007f9bdf5bbcdf in __GI___pthread_mutex_lock
>  #3  0x000055a6f8551447 in qemu_mutex_lock_impl
>  #4  0x000055a6f854be77 in co_schedule_bh_cb
>  #5  0x000055a6f854b781 in aio_bh_poll
>  #6  0x000055a6f854b781 in aio_bh_poll
>  #7  0x000055a6f854f01b in aio_poll
>  #8  0x000055a6f825a488 in iothread_run
>  #9  0x000055a6f8551306 in qemu_thread_start
>  #10 0x00007f9bdf5b9dd5 in start_thread
>  #11 0x00007f9bdf2e2ead in clone
> 
>  (gdb) thread 8
>  [Switching to thread 8 (Thread 0x7f9bd6dae700 (LWP 90467))]
>  #3  0x000055a6f8551447 in qemu_mutex_lock_impl (mutex=0x55a6fac8fea0,
>      file=0x55a6f8730c3f "util/async.c", line=511) at util/qemu-thread-posix.c:66
>  66	    err = pthread_mutex_lock(&mutex->lock);
>  (gdb) up 3
>  #3  0x000055a6f8551447 in qemu_mutex_lock_impl (mutex=0x55a6fac8fea0,
>      file=0x55a6f8730c3f "util/async.c", line=511) at util/qemu-thread-posix.c:66
>  66	    err = pthread_mutex_lock(&mutex->lock);
>  (gdb) p mutex.lock.__data.__owner
>  $6 = 90472
> 
> Signed-off-by: Sergio Lopez <slp@redhat.com>
> ---
>  hw/block/dataplane/virtio-blk.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
> index 8c37bd314a..358e6ae89b 100644
> --- a/hw/block/dataplane/virtio-blk.c
> +++ b/hw/block/dataplane/virtio-blk.c
> @@ -280,12 +280,11 @@ void virtio_blk_data_plane_stop(VirtIODevice *vdev)
>  
>      aio_context_acquire(s->ctx);
>      aio_wait_bh_oneshot(s->ctx, virtio_blk_data_plane_stop_bh, s);
> +    aio_context_release(s->ctx);
>  
>      /* Drain and switch bs back to the QEMU main loop */
>      blk_set_aio_context(s->conf->conf.blk, qemu_get_aio_context());
>  
> -    aio_context_release(s->ctx);
> -

blk_set_aio_context() requires that the caller hold the lock for the
source context, so I'm afraid this is wrong.

However, I think the problem might already be fixed with my "block: Use
normal drain for bdrv_set_aio_context()" (contained in a pull request,
which isn't merged yet), which makes bdrv_set_aio_context() use a real
drain operation. This way, the requests of the mirror jobs should be
already drained before we even call into block_job_detach_aio_context().

Can you give this a try and see whether it fixes your problem?

Kevin