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. > > #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. 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? I don't really have an idea immediately, we'd have to find out where the stuck I/O stops being processed. > > But I think we're actually not very far from removing the AioContext > > lock, so if we can't find an easy fix in the meantime, waiting for that > > could be a realistic option. > > > > Glad to hear :) Do you think it will be in time for QEMU 8.2? Otherwise, > I'll go ahead and send what I have for fixing the deadlocks from this > mail thread in the following days. The stuck guest IO can happen even > without any of those changes (on current master, i.e. > ebca80bbdb5c1650e4b753a3d13b43634e7dfe05, at least when starting a > mirror job). Having it in 8.2 is certainly the plan, but plans don't always work out. If you have fixes that aren't too ugly, we can still apply them. Kevin