On Thu, Feb 28, 2019 at 06:22:02PM +0100, Kevin Wolf wrote: > Am 28.02.2019 um 18:04 hat Sergio Lopez geschrieben: > > On Thu, Feb 28, 2019 at 04:50:53PM +0100, Kevin Wolf wrote: > > > Am 28.02.2019 um 16:01 hat Sergio Lopez geschrieben: > > > > On Wed, Feb 27, 2019 at 06:37:14PM +0100, Kevin Wolf wrote: > > > > > 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 <s...@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. > > > > > > > > TBH, I was quite sure this patch was wrong myself, but I thought it was > > > > still a good way to illustrate the issue. > > > > > > > > > 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? > > > > > > > > I've applied your patchset to my local copy, but it doesn't fix the > > > > issue. > > > > > > > > The problem is the coroutine is already scheduled to be run in the > > > > iothread context, which means job->busy == true, so we can't switch to > > > > it from any other place. > > > > > > I still don't understand this because with job->paused == true the > > > hanging loop wouldn't even be started. And after bdrv_drained_begin() > > > returns, all jobs that use the node in question should be paused (see > > > child_job_drained_begin/poll). > > > > > > So somehow that drain (called in bdrv_set_aio_context()) doesn't seem to > > > fully do what it is supposed to do? > > > > IIUC, child_job_drained_begin() requests the job to be paused (something > > that block_job_detach_aio_context() also does), but we don't get to > > child_job_drained_poll() as bdrv_parent_drained_begin_single() is called > > with "poll == false" by bdrv_parent_drained_begin(). > > > > But even if we did, that probably won't help in some scenarios, as > > mirror's drained_poll implementation just checks if there are in_flight > > requests, so it might happen that BDRV_POLL_WHILE returns without having > > called aio_wait() a single time. > > > > In other words, I think the drain code makes sure there aren't any > > in_flight requests in the chain, but doesn't provide by itself a > > guarantee that the jobs have been paused. > > Yes, seems this is what it does. But I think that's not enough because > if the job isn't paused, it can still issue new requests. > > I'm not sure whether mirror_drained_poll() or child_job_drained_poll() > is to blame, but the logic is wrong there: We should only return that > the job is quiescent when it has reached a pause point _and_ > s->in_flight == 0.
If we expect this to be the case even when bdrv_parent_drained_begin_single() is called from bdrv_parent_drained_begin() with poll == false, then we need to make the job_pause() at child_job_drained_begin() to take effect immeditaly, probaly putting an AIO_WAIT_WHILE afterwards. Otherwise, we can simply add an extra condition at child_job_drained_poll(), before the drv->drained_poll(), to return true if the job isn't yet paused. Thanks, Sergio (slp).