I/O requests hang after stop/cont commands at least since QEMU 2.10.0 with -drive iops=100:
(guest)$ dd if=/dev/zero of=/dev/vdb oflag=direct count=1000 (qemu) stop (qemu) cont ...I/O is stuck... This happens because blk_set_aio_context() detaches the ThrottleState while requests may still be in flight: if (tgm->throttle_state) { throttle_group_detach_aio_context(tgm); throttle_group_attach_aio_context(tgm, new_context); } This patch encloses the detach/attach calls in a drained region so no I/O request is left hanging. Also add assertions so we don't make the same mistake again in the future. Reported-by: Yongxue Hong <yh...@redhat.com> Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com> --- v4: * Simplified patch in response to Berto's review --- block/block-backend.c | 2 ++ block/throttle-groups.c | 6 ++++++ 2 files changed, 8 insertions(+) diff --git a/block/block-backend.c b/block/block-backend.c index 45d9101be3..da2f6c0f8a 100644 --- a/block/block-backend.c +++ b/block/block-backend.c @@ -1748,8 +1748,10 @@ void blk_set_aio_context(BlockBackend *blk, AioContext *new_context) if (bs) { if (tgm->throttle_state) { + bdrv_drained_begin(bs); throttle_group_detach_aio_context(tgm); throttle_group_attach_aio_context(tgm, new_context); + bdrv_drained_end(bs); } bdrv_set_aio_context(bs, new_context); } diff --git a/block/throttle-groups.c b/block/throttle-groups.c index b291a88481..2587f19ca3 100644 --- a/block/throttle-groups.c +++ b/block/throttle-groups.c @@ -594,6 +594,12 @@ void throttle_group_attach_aio_context(ThrottleGroupMember *tgm, void throttle_group_detach_aio_context(ThrottleGroupMember *tgm) { ThrottleTimers *tt = &tgm->throttle_timers; + + /* Requests must have been drained */ + assert(tgm->pending_reqs[0] == 0 && tgm->pending_reqs[1] == 0); + assert(qemu_co_queue_empty(&tgm->throttled_reqs[0])); + assert(qemu_co_queue_empty(&tgm->throttled_reqs[1])); + throttle_timers_detach_aio_context(tt); tgm->aio_context = NULL; } -- 2.13.6