Am 18.10.2023 um 11:42 hat Fiona Ebner geschrieben: > Am 17.10.23 um 16:20 schrieb Kevin Wolf: > > Am 17.10.2023 um 15:37 hat Fiona Ebner geschrieben: > >> Am 17.10.23 um 14:12 schrieb Kevin Wolf: > >>> Am 17.10.2023 um 12:18 hat Fiona Ebner geschrieben: > >>>> I ran into similar issues now with mirror, (both deadlocks and stuck > >>>> guest IO at other times), and interestingly, also during job start. > >>>> > >>>> Also had a backtrace similar to [0] once, so I took a closer look. > >>>> Probably was obvious to others already, but for the record: > >>>> > >>>> 1. the graph is locked by the main thread > >>>> 2. the iothread holds the AioContext lock > >>>> 3. the main thread waits on the AioContext lock > >>>> 4. the iothread waits for coroutine spawned by blk_is_available() > >>> > >>> Where does this blk_is_available() in the iothread come from? Having it > >>> wait without dropping the AioContext lock sounds like something that > >>> we'd want to avoid. Ideally, devices using iothreads shouldn't use > >>> synchronous requests at all, but I think scsi-disk might have some of > >>> them. > >>> > >> > >> It's part of the request handling in virtio-scsi: > >> > >>> #0 0x00007ff7f5f55136 in __ppoll (fds=0x7ff7e40030c0, nfds=8, > >>> timeout=<optimized out>, sigmask=0x0) at > >>> ../sysdeps/unix/sysv/linux/ppoll.c:42 > >>> #1 0x00005587132615ab in qemu_poll_ns (fds=0x7ff7e40030c0, nfds=8, > >>> timeout=-1) at ../util/qemu-timer.c:339 > >>> #2 0x000055871323e8b1 in fdmon_poll_wait (ctx=0x55871598d5e0, > >>> ready_list=0x7ff7f288ebe0, timeout=-1) at ../util/fdmon-poll.c:79 > >>> #3 0x000055871323e1ed in aio_poll (ctx=0x55871598d5e0, blocking=true) at > >>> ../util/aio-posix.c:670 > >>> #4 0x0000558713089efa in bdrv_poll_co (s=0x7ff7f288ec90) at > >>> /home/febner/repos/qemu/block/block-gen.h:43 > >>> #5 0x000055871308c362 in blk_is_available (blk=0x55871599e2f0) at > >>> block/block-gen.c:1426 > >>> #6 0x0000558712f6843b in virtio_scsi_ctx_check (s=0x558716c049c0, > >>> d=0x55871581cd30) at ../hw/scsi/virtio-scsi.c:290 > > > > Oh... So essentially for an assertion. > > > > I wonder if the blk_is_available() check introduced in 2a2d69f490c is > > even necessary any more, because BlockBackend has its own AioContext > > now. And if blk_bs(blk) != NULL isn't what we actually want to check if > > the check is necessary, because calling bdrv_is_inserted() doesn't seem > > to have been intended. blk_bs() wouldn't have to poll. > > > > Could virtio_scsi_hotunplug() be an issue with removing or modifying > the check? There's a call there which sets the blk's AioContext to > qemu_get_aio_context(). Or are we sure that the assert in > virtio_scsi_ctx_check() can't be reached after that?
I think that would be the kind of bug that the assertion tries to catch, because then we would be sending requests to blk from a thread that doesn't match its AioContext (which will be allowed soon, but not quite yet). Before resetting the AioContext, virtio_scsi_hotunplug() calls qdev_simple_device_unplug_cb(), which unrealizes the SCSI device. This calls scsi_qdev_unrealize() -> scsi_device_purge_requests(), which in turn drains blk and cancels all pending requests. So there should be nothing left that could call into virtio_scsi_ctx_check() any more. The other argument is that after unrealize, virtio_scsi_device_get() would return NULL anyway, so even if a request were still pending, it would just fail instead of accessing the unplugged device. > >>> #7 0x0000558712f697e4 in virtio_scsi_handle_cmd_req_prepare > >>> (s=0x558716c049c0, req=0x7ff7e400b650) at ../hw/scsi/virtio-scsi.c:788 > >>> #8 0x0000558712f699b0 in virtio_scsi_handle_cmd_vq (s=0x558716c049c0, > >>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:831 > >>> #9 0x0000558712f69bcb in virtio_scsi_handle_cmd (vdev=0x558716c049c0, > >>> vq=0x558716c0d2a8) at ../hw/scsi/virtio-scsi.c:867 > >>> #10 0x0000558712f96812 in virtio_queue_notify_vq (vq=0x558716c0d2a8) at > >>> ../hw/virtio/virtio.c:2263 > >>> #11 0x0000558712f99b75 in virtio_queue_host_notifier_read > >>> (n=0x558716c0d31c) at ../hw/virtio/virtio.c:3575 > >>> #12 0x000055871323d8b5 in aio_dispatch_handler (ctx=0x55871598d5e0, > >>> node=0x558716771000) at ../util/aio-posix.c:372 > >>> #13 0x000055871323d988 in aio_dispatch_ready_handlers > >>> (ctx=0x55871598d5e0, ready_list=0x7ff7f288eeb0) at ../util/aio-posix.c:401 > >> > >> > >>>> As for why it doesn't progress, blk_co_is_available_entry() uses > >>>> bdrv_graph_co_rdlock() and can't get it, because the main thread has the > >>>> write lock. Should be fixed once the AioContext locks are gone, but not > >>>> sure what should be done to avoid it until then. > >>> > >>> Then the nested event loop in blk_is_available() would probably be > >>> enough to make progress, yes. > >>> > >>> Maybe we could actually drop the lock (and immediately reacquire it) in > >>> AIO_WAIT_WHILE() even if we're in the home thread? That should give the > >>> main thread a chance to make progress. > >> > >> Seems to work :) I haven't run into the issue with the following change > >> anymore, but I have to say, running into that specific deadlock only > >> happened every 10-15 tries or so before. Did 30 tests now. But > >> unfortunately, the stuck IO issue is still there. > >> > >>> diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h > >>> index 5449b6d742..da159501ca 100644 > >>> --- a/include/block/aio-wait.h > >>> +++ b/include/block/aio-wait.h > >>> @@ -88,7 +88,13 @@ extern AioWait global_aio_wait; > >>> smp_mb__after_rmw(); \ > >>> if (ctx_ && in_aio_context_home_thread(ctx_)) { \ > >>> while ((cond)) { \ > >>> + if (unlock && ctx_) { \ > >>> + aio_context_release(ctx_); \ > >>> + } \ > >>> aio_poll(ctx_, true); \ > >>> + if (unlock && ctx_) { \ > >>> + aio_context_acquire(ctx_); \ > >>> + } \ > >>> waited_ = true; \ > >>> } \ > >>> } else { \ > > > > For reacquiring the lock, I really meant "immediately". Calling > > aio_poll() without the lock is wrong. > > Unfortunately, then it's not enough, because the call to aio_poll() is > blocking and because the lock is held during that call, the very same > deadlock can still happen. Oh, right. What we would need is an aio_poll() that drops the lock while it blocks, pretty much like the main loop does with the BQL. I seem to remember that this was an intentional difference, but I don't remember why. Paolo, I'm sure you remember? > > > > What does the stuck I/O look like? Is it stuck in the backend, i.e. the > > device started requests that never complete? Or stuck from the guest > > perspective, i.e. the device never checks for new requests? > > > > AFAICT, from the guest perspective. > > > I don't really have an idea immediately, we'd have to find out where the > > stuck I/O stops being processed. > > > > I've described it in an earlier mail in this thread: > https://lists.nongnu.org/archive/html/qemu-devel/2023-10/msg01900.html > > Quoting from there: > > > After the IO was stuck in the guest, I used bdrv_next_all_states() to > > iterate over the states and there's only the bdrv_raw and the > > bdrv_host_device. For both, tracked_requests was empty. And bs->in_flight and blk->in_flight are 0, too? Is anything quiesced? > > What is also very interesting is that the IO isn't always dead > > immediately. It can be that the fio command still runs with lower speed > > for a while (sometimes even up to about a minute, but most often about > > 10-15 seconds or so). During that time, I still can see calls to > > virtio_scsi_handle_cmd() and blk_aio_write_entry(). Then they suddenly stop. > > Noting again that (at least for backup) it happens with both virtio-blk > and virtio-scsi and with both aio=io_uring and aio=threads. I also tried > different host kernels 5.15, 6.2 and 6.5 and guest kernels 5.10 and 6.1. You say "at least for backup". Did you see the bug for other job types, too? Kevin