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Re: [Qemu-block] [PATCH] virtio-blk: dataplane: release AioContext befor
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-block] [PATCH] virtio-blk: dataplane: release AioContext before blk_set_aio_context |
Date: |
Wed, 27 Feb 2019 18:37:14 +0100 |
User-agent: |
Mutt/1.11.3 (2019-02-01) |
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 <address@hidden>
> ---
> 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