Am 02.05.2025 um 19:52 hat Denis V. Lunev geschrieben:
> On 5/2/25 19:34, Kevin Wolf wrote:
> > Am 24.04.2025 um 19:32 hat Andrey Drobyshev geschrieben:
> > > Hi all,
> > >
> > > There's a bug in block layer which leads to block graph deadlock.
> > > Notably, it takes place when blockdev IO is processed within a separate
> > > iothread.
> > >
> > > This was initially caught by our tests, and I was able to reduce it to a
> > > relatively simple reproducer. Such deadlocks are probably supposed to
> > > be covered in iotests/graph-changes-while-io, but this deadlock isn't.
> > >
> > > Basically what the reproducer does is launches QEMU with a drive having
> > > 'iothread' option set, creates a chain of 2 snapshots, launches
> > > block-commit job for a snapshot and then dismisses the job, starting
> > > from the lower snapshot. If the guest is issuing IO at the same time,
> > > there's a race in acquiring block graph lock and a potential deadlock.
> > >
> > > Here's how it can be reproduced:
> > >
> > > 1. Run QEMU:
> > > > SRCDIR=/path/to/srcdir
> > > > $SRCDIR/build/qemu-system-x86_64 -enable-kvm \
> > > > -machine q35 -cpu Nehalem \
> > > > -name guest=alma8-vm,debug-threads=on \
> > > > -m 2g -smp 2 \
> > > > -nographic -nodefaults \
> > > > -qmp unix:/var/run/alma8-qmp.sock,server=on,wait=off \
> > > > -serial unix:/var/run/alma8-serial.sock,server=on,wait=off \
> > > > -object iothread,id=iothread0 \
> > > > -blockdev
> > > > node-name=disk,driver=qcow2,file.driver=file,file.filename=/path/to/img/alma8.qcow2
> > > > \
> > > > -device virtio-blk-pci,drive=disk,iothread=iothread0
> > > 2. Launch IO (random reads) from within the guest:
> > > > nc -U /var/run/alma8-serial.sock
> > > > ...
> > > > [root@alma8-vm ~]# fio --name=randread --ioengine=libaio --direct=1
> > > > --bs=4k --size=1G --numjobs=1 --time_based=1 --runtime=300
> > > > --group_reporting --rw=randread --iodepth=1 --filename=/testfile
> > > 3. Run snapshots creation & removal of lower snapshot operation in a
> > > loop (script attached):
> > > > while /bin/true ; do ./remove_lower_snap.sh ; done
> > > And then it occasionally hangs.
> > >
> > > Note: I've tried bisecting this, and looks like deadlock occurs starting
> > > from the following commit:
> > >
> > > (BAD) 5bdbaebcce virtio: Re-enable notifications after drain
> > > (GOOD) c42c3833e0 virtio-scsi: Attach event vq notifier with no_poll
> > >
> > > On the latest v10.0.0 it does hang as well.
> > >
> > >
> > > Here's backtrace of the main thread:
> > >
> > > > #0 0x00007fc547d427ce in __ppoll (fds=0x557eb79657b0, nfds=1,
> > > > timeout=<optimized out>, sigmask=0x0) at
> > > > ../sysdeps/unix/sysv/linux/ppoll.c:43
> > > > #1 0x0000557eb47d955c in qemu_poll_ns (fds=0x557eb79657b0, nfds=1,
> > > > timeout=-1) at ../util/qemu-timer.c:329
> > > > #2 0x0000557eb47b2204 in fdmon_poll_wait (ctx=0x557eb76c5f20,
> > > > ready_list=0x7ffd94b4edd8, timeout=-1) at ../util/fdmon-poll.c:79
> > > > #3 0x0000557eb47b1c45 in aio_poll (ctx=0x557eb76c5f20, blocking=true)
> > > > at ../util/aio-posix.c:730
> > > > #4 0x0000557eb4621edd in bdrv_do_drained_begin (bs=0x557eb795e950,
> > > > parent=0x0, poll=true) at ../block/io.c:378
> > > > #5 0x0000557eb4621f7b in bdrv_drained_begin (bs=0x557eb795e950) at
> > > > ../block/io.c:391
> > > > #6 0x0000557eb45ec125 in bdrv_change_aio_context (bs=0x557eb795e950,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7682
> > > > #7 0x0000557eb45ebf2b in bdrv_child_change_aio_context
> > > > (c=0x557eb7964250, ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7608
> > > > #8 0x0000557eb45ec0c4 in bdrv_change_aio_context (bs=0x557eb79575e0,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7668
> > > > #9 0x0000557eb45ebf2b in bdrv_child_change_aio_context
> > > > (c=0x557eb7e59110, ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7608
> > > > #10 0x0000557eb45ec0c4 in bdrv_change_aio_context (bs=0x557eb7e51960,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7668
> > > > #11 0x0000557eb45ebf2b in bdrv_child_change_aio_context
> > > > (c=0x557eb814ed80, ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7608
> > > > #12 0x0000557eb45ee8e4 in child_job_change_aio_ctx (c=0x557eb7c9d3f0,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../blockjob.c:157
> > > > #13 0x0000557eb45ebe2d in bdrv_parent_change_aio_context
> > > > (c=0x557eb7c9d3f0, ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7592
> > > > #14 0x0000557eb45ec06b in bdrv_change_aio_context (bs=0x557eb7d74310,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7661
> > > > #15 0x0000557eb45dcd7e in bdrv_child_cb_change_aio_ctx
> > > > (child=0x557eb8565af0, ctx=0x557eb76c5f20, visited=0x557eb7e06b60
> > > > = {...}, tran=0x557eb7a87160, errp=0x0) at ../block.c:1234
> > > > #16 0x0000557eb45ebe2d in bdrv_parent_change_aio_context
> > > > (c=0x557eb8565af0, ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7592
> > > > #17 0x0000557eb45ec06b in bdrv_change_aio_context (bs=0x557eb79575e0,
> > > > ctx=0x557eb76c5f20, visited=0x557eb7e06b60 = {...},
> > > > tran=0x557eb7a87160, errp=0x0)
> > > > at ../block.c:7661
> > > > #18 0x0000557eb45ec1f3 in bdrv_try_change_aio_context
> > > > (bs=0x557eb79575e0, ctx=0x557eb76c5f20, ignore_child=0x0, errp=0x0) at
> > > > ../block.c:7715
> > > > #19 0x0000557eb45e1b15 in bdrv_root_unref_child (child=0x557eb7966f30)
> > > > at ../block.c:3317
> > > > #20 0x0000557eb45eeaa8 in block_job_remove_all_bdrv
> > > > (job=0x557eb7952800) at ../blockjob.c:209
> > > > #21 0x0000557eb45ee641 in block_job_free (job=0x557eb7952800) at
> > > > ../blockjob.c:82
> > > > #22 0x0000557eb45f17af in job_unref_locked (job=0x557eb7952800) at
> > > > ../job.c:474
> > > > #23 0x0000557eb45f257d in job_do_dismiss_locked (job=0x557eb7952800) at
> > > > ../job.c:771
> > > > #24 0x0000557eb45f25fe in job_dismiss_locked (jobptr=0x7ffd94b4f400,
> > > > errp=0x7ffd94b4f488) at ../job.c:783
> > > > --Type <RET> for more, q to quit, c to continue without paging--
> > > > #25 0x0000557eb45d8e84 in qmp_job_dismiss (id=0x557eb7aa42b0
> > > > "commit-snap1", errp=0x7ffd94b4f488) at ../job-qmp.c:138
> > > > #26 0x0000557eb472f6a3 in qmp_marshal_job_dismiss (args=0x7fc52c00a3b0,
> > > > ret=0x7fc53c880da8, errp=0x7fc53c880da0) at qapi/qapi-commands-job.c:221
> > > > #27 0x0000557eb47a35f3 in do_qmp_dispatch_bh (opaque=0x7fc53c880e40) at
> > > > ../qapi/qmp-dispatch.c:128
> > > > #28 0x0000557eb47d1cd2 in aio_bh_call (bh=0x557eb79568f0) at
> > > > ../util/async.c:172
> > > > #29 0x0000557eb47d1df5 in aio_bh_poll (ctx=0x557eb76c0200) at
> > > > ../util/async.c:219
> > > > #30 0x0000557eb47b12f3 in aio_dispatch (ctx=0x557eb76c0200) at
> > > > ../util/aio-posix.c:436
> > > > #31 0x0000557eb47d2266 in aio_ctx_dispatch (source=0x557eb76c0200,
> > > > callback=0x0, user_data=0x0) at ../util/async.c:361
> > > > #32 0x00007fc549232f4f in g_main_dispatch (context=0x557eb76c6430) at
> > > > ../glib/gmain.c:3364
> > > > #33 g_main_context_dispatch (context=0x557eb76c6430) at
> > > > ../glib/gmain.c:4079
> > > > #34 0x0000557eb47d3ab1 in glib_pollfds_poll () at
> > > > ../util/main-loop.c:287
> > > > #35 0x0000557eb47d3b38 in os_host_main_loop_wait (timeout=0) at
> > > > ../util/main-loop.c:310
> > > > #36 0x0000557eb47d3c58 in main_loop_wait (nonblocking=0) at
> > > > ../util/main-loop.c:589
> > > > #37 0x0000557eb4218b01 in qemu_main_loop () at ../system/runstate.c:835
> > > > #38 0x0000557eb46df166 in qemu_default_main (opaque=0x0) at
> > > > ../system/main.c:50
> > > > #39 0x0000557eb46df215 in main (argc=24, argv=0x7ffd94b4f8d8) at
> > > > ../system/main.c:80
> > >
> > > And here's coroutine trying to acquire read lock:
> > >
> > > > (gdb) qemu coroutine reader_queue->entries.sqh_first
> > > > #0 0x0000557eb47d7068 in qemu_coroutine_switch (from_=0x557eb7aa48b0,
> > > > to_=0x7fc537fff508, action=COROUTINE_YIELD) at
> > > > ../util/coroutine-ucontext.c:321
> > > > #1 0x0000557eb47d4d4a in qemu_coroutine_yield () at
> > > > ../util/qemu-coroutine.c:339
> > > > #2 0x0000557eb47d56c8 in qemu_co_queue_wait_impl (queue=0x557eb59954c0
> > > > <reader_queue>, lock=0x7fc53c57de50, flags=0) at
> > > > ../util/qemu-coroutine-lock.c:60
> > > > #3 0x0000557eb461fea7 in bdrv_graph_co_rdlock () at
> > > > ../block/graph-lock.c:231
> > > > #4 0x0000557eb460c81a in graph_lockable_auto_lock (x=0x7fc53c57dee3)
> > > > at /home/root/src/qemu/master/include/block/graph-lock.h:213
> > > > #5 0x0000557eb460fa41 in blk_co_do_preadv_part
> > > > (blk=0x557eb84c0810, offset=6890553344, bytes=4096,
> > > > qiov=0x7fc530006988, qiov_offset=0, flags=BDRV_REQ_REGISTERED_BUF) at
> > > > ../block/block-backend.c:1339
> > > > #6 0x0000557eb46104d7 in blk_aio_read_entry (opaque=0x7fc530003240) at
> > > > ../block/block-backend.c:1619
> > > > #7 0x0000557eb47d6c40 in coroutine_trampoline (i0=-1213577040,
> > > > i1=21886) at ../util/coroutine-ucontext.c:175
> > > > #8 0x00007fc547c2a360 in __start_context () at
> > > > ../sysdeps/unix/sysv/linux/x86_64/__start_context.S:91
> > > > #9 0x00007ffd94b4ea40 in ()
> > > > #10 0x0000000000000000 in ()
> > >
> > > So it looks like main thread is processing job-dismiss request and is
> > > holding write lock taken in block_job_remove_all_bdrv() (frame #20
> > > above). At the same time iothread spawns a coroutine which performs IO
> > > request. Before the coroutine is spawned, blk_aio_prwv() increases
> > > 'in_flight' counter for Blk. Then blk_co_do_preadv_part() (frame #5) is
> > > trying to acquire the read lock. But main thread isn't releasing the
> > > lock as blk_root_drained_poll() returns true since blk->in_flight > 0.
> > > Here's the deadlock.
> > >
> > > Any comments and suggestions on the subject are welcomed. Thanks!
> > I think this is what the blk_wait_while_drained() call was supposed to
> > address in blk_co_do_preadv_part(). However, with the use of multiple
> > I/O threads, this is racy.
> >
> > Do you think that in your case we hit the small race window between the
> > checks in blk_wait_while_drained() and GRAPH_RDLOCK_GUARD()? Or is there
> > another reason why blk_wait_while_drained() didn't do its job?
> >
> At my opinion there is very big race window. Main thread has
> eaten graph write lock. After that another coroutine is stalled
> within GRAPH_RDLOCK_GUARD() as there is no drain at the moment and only
> after that main thread has started drain.
You're right, I confused taking the write lock with draining there.
> That is why Fiona's idea is looking working. Though this would mean
> that normally we should always do that at the moment when we acquire
> write lock. May be even inside this function.
I actually see now that not all of my graph locking patches were merged.
At least I did have the thought that bdrv_drained_begin() must be marked
GRAPH_UNLOCKED because it polls. That means that calling it from inside
bdrv_try_change_aio_context() is actually forbidden (and that's the part
I didn't see back then because it doesn't have TSA annotations).
If you refactor the code to move the drain out to before the lock is
taken, I think you end up with Fiona's patch, except you'll remove the
forbidden inner drain and add more annotations for some functions and
clarify the rules around them. I don't know, but I wouldn't be surprised
if along the process we find other bugs, too.
So Fiona's drain looks right to me, but we should probably approach it
more systematically.
Kevin
