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Re: virtio-scsi and another complex AioContext issue
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From: |
Stefan Hajnoczi |
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Subject: |
Re: virtio-scsi and another complex AioContext issue |
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Date: |
Tue, 23 Jun 2020 15:24:54 +0100 |
On Mon, Jun 22, 2020 at 04:16:04PM +0200, Sergio Lopez wrote:
> On Fri, Jun 19, 2020 at 02:04:06PM +0100, Stefan Hajnoczi wrote:
> > On Thu, Jun 11, 2020 at 10:36:22AM +0200, Sergio Lopez wrote:
> > > Hi,
> > >
> > > While debugging BZ#1844343, I managed to reproduce the issue which
> > > leads to crash with a backtrace like this one:
> > >
> > > <---- snip ---->
> > > Thread 2 (Thread 0x7fe208463f00 (LWP 1659571)):
> > > #0 0x00007fe2033b78ed in __lll_lock_wait () at /lib64/libpthread.so.0
> > > #1 0x00007fe2033b0bd4 in pthread_mutex_lock () at /lib64/libpthread.so.0
> > > #2 0x0000560caa8f1e6d in qemu_mutex_lock_impl
> > > (mutex=0x560cacc68a10, file=0x560caaa9797f "util/async.c", line=521)
> > > at util/qemu-thread-posix.c:78
> > > #3 0x0000560caa82414d in bdrv_set_aio_context_ignore
> > > (bs=bs@entry=0x560cacc73570,
> > > new_context=new_context@entry=0x560cacc5fed0,
> > > ignore=ignore@entry=0x7ffe388b1cc0) at block.c:6192
> > > #4 0x0000560caa824503 in bdrv_child_try_set_aio_context
> > > (bs=bs@entry=0x560cacc73570, ctx=0x560cacc5fed0,
> > > ignore_child=<optimized out>, errp=<optimized out>)
> > > at block.c:6272
> > > #5 0x0000560caa859e6b in blk_do_set_aio_context
> > > (blk=0x560cacecf370, new_context=0x560cacc5fed0,
> > > update_root_node=update_root_node@entry=true, errp=errp@entry=0x0) at
> > > block/block-backend.c:1989
> > > #6 0x0000560caa85c501 in blk_set_aio_context
> > > (blk=<optimized out>, new_context=<optimized out>,
> > > errp=errp@entry=0x0) at block/block-backend.c:2010
> > > #7 0x0000560caa61db30 in virtio_scsi_hotunplug
> > > (hotplug_dev=0x560cadaafbd0, dev=0x560cacec1210, errp=0x7ffe388b1d80)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/scsi/virtio-scsi.c:869
> > > #8 0x0000560caa6ccd1e in qdev_unplug (dev=0x560cacec1210,
> > > errp=errp@entry=0x7ffe388b1db8)
> > > at qdev-monitor.c:872
> > > #9 0x0000560caa6ccd9e in qmp_device_del (id=<optimized out>,
> > > errp=errp@entry=0x7ffe388b1db8)
> > > at qdev-monitor.c:884
> > > #10 0x0000560caa7ec4d3 in qmp_marshal_device_del
> > > (args=<optimized out>, ret=<optimized out>, errp=0x7ffe388b1e18) at
> > > qapi/qapi-commands-qdev.c:99
> > > #11 0x0000560caa8a45ec in do_qmp_dispatch
> > > (errp=0x7ffe388b1e10, allow_oob=<optimized out>, request=<optimized
> > > out>, cmds=0x560cab1928a0 <qmp_commands>) at qapi/qmp-dispatch.c:132
> > > #12 0x0000560caa8a45ec in qmp_dispatch
> > > (cmds=0x560cab1928a0 <qmp_commands>, request=<optimized out>,
> > > allow_oob=<optimized out>)
> > > at qapi/qmp-dispatch.c:175
> > > #13 0x0000560caa7c2521 in monitor_qmp_dispatch (mon=0x560cacca2f00,
> > > req=<optimized out>)
> > > at monitor/qmp.c:145
> > > #14 0x0000560caa7c2bba in monitor_qmp_bh_dispatcher (data=<optimized
> > > out>) at monitor/qmp.c:234
> > > #15 0x0000560caa8ec716 in aio_bh_call (bh=0x560cacbd80e0) at
> > > util/async.c:117
> > > #16 0x0000560caa8ec716 in aio_bh_poll (ctx=ctx@entry=0x560cacbd6da0) at
> > > util/async.c:117
> > > #17 0x0000560caa8efb04 in aio_dispatch (ctx=0x560cacbd6da0) at
> > > util/aio-posix.c:459
> > > #18 0x0000560caa8ec5f2 in aio_ctx_dispatch
> > > (source=<optimized out>, callback=<optimized out>,
> > > user_data=<optimized out>) at util/async.c:260
> > > #19 0x00007fe2078d167d in g_main_context_dispatch () at
> > > /lib64/libglib-2.0.so.0
> > > #20 0x0000560caa8eebb8 in glib_pollfds_poll () at util/main-loop.c:219
> > > #21 0x0000560caa8eebb8 in os_host_main_loop_wait (timeout=<optimized
> > > out>) at util/main-loop.c:242
> > > #22 0x0000560caa8eebb8 in main_loop_wait (nonblocking=<optimized out>) at
> > > util/main-loop.c:518
> > > #23 0x0000560caa6cfe51 in main_loop () at vl.c:1828
> > > #24 0x0000560caa57b322 in main (argc=<optimized out>, argv=<optimized
> > > out>, envp=<optimized out>)
> > > at vl.c:4504
> > >
> > > Thread 1 (Thread 0x7fe1fb059700 (LWP 1659573)):
> > > #0 0x00007fe20301b70f in raise () at /lib64/libc.so.6
> > > #1 0x00007fe203005b25 in abort () at /lib64/libc.so.6
> > > #2 0x00007fe2030059f9 in _nl_load_domain.cold.0 () at /lib64/libc.so.6
> > > #3 0x00007fe203013cc6 in .annobin_assert.c_end () at /lib64/libc.so.6
> > > #4 0x0000560caa85bfe4 in blk_get_aio_context (blk=0x560cacecf370) at
> > > block/block-backend.c:1968
> > > #5 0x0000560caa85bfe4 in blk_get_aio_context (blk=0x560cacecf370) at
> > > block/block-backend.c:1962
> > > #6 0x0000560caa61d79c in virtio_scsi_ctx_check (s=0x560cadaafbd0,
> > > s=0x560cadaafbd0, d=0x560cacec1210)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/scsi/virtio-scsi.c:250
> > > #7 0x0000560caa61d79c in virtio_scsi_handle_cmd_req_prepare
> > > (req=0x7fe1ec013880, s=0x560cadaafbd0)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/scsi/virtio-scsi.c:569
> > > #8 0x0000560caa61d79c in virtio_scsi_handle_cmd_vq
> > > (s=s@entry=0x560cadaafbd0, vq=vq@entry=0x7fe1f82ac140)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/scsi/virtio-scsi.c:612
> > > #9 0x0000560caa61e48e in virtio_scsi_data_plane_handle_cmd
> > > (vdev=<optimized out>, vq=0x7fe1f82ac140)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/scsi/virtio-scsi-dataplane.c:60
> > > #10 0x0000560caa62bfbe in virtio_queue_notify_aio_vq (vq=<optimized out>)
> > > at
> > > /usr/src/debug/qemu-kvm-4.2.0-22.module+el8.2.1+6758+cb8d64c2.x86_64/hw/virtio/virtio.c:2243
> > > #11 0x0000560caa8ef046 in run_poll_handlers_once
> > > (ctx=ctx@entry=0x560cacc689b0, timeout=timeout@entry=0x7fe1fb058658)
> > > at util/aio-posix.c:517
> > > #12 0x0000560caa8efbc5 in try_poll_mode (timeout=0x7fe1fb058658,
> > > ctx=0x560cacc689b0)
> > > at util/aio-posix.c:607
> > > #13 0x0000560caa8efbc5 in aio_poll (ctx=0x560cacc689b0,
> > > blocking=blocking@entry=true)
> > > at util/aio-posix.c:639
> > > #14 0x0000560caa6ca7f4 in iothread_run (opaque=0x560cacc1f000) at
> > > iothread.c:75
> > > #15 0x0000560caa8f1d84 in qemu_thread_start (args=0x560cacc666f0) at
> > > util/qemu-thread-posix.c:519
> > > #16 0x00007fe2033ae2de in start_thread () at /lib64/libpthread.so.0
> > > #17 0x00007fe2030dfe83 in clone () at /lib64/libc.so.6
> > > <---- snip ---->
> > >
> > > Both the code path initiated by virtio_scsi_data_plane_cmd_vq() and
> > > the one coming down from virtio_scsi_hotunplug() should be protected
> > > by virtio_scsi_acquire(), but this can still happen because the latter
> > > works by acquiring the AioContext pointed by s->ctx, and we have this
> > > in bdrv_set_aio_context_ignore():
> > >
> > > 6140 void bdrv_set_aio_context_ignore(BlockDriverState *bs,
> > > 6141 AioContext *new_context, GSList
> > > **ignore)
> > > 6142 {
> > > (...)
> > > 6179 /*
> > > 6180 * If this function was recursively called from
> > > 6181 * bdrv_set_aio_context_ignore(), there may be nodes in the
> > > 6182 * subtree that have not yet been moved to the new AioContext.
> > > 6183 * Release the old one so bdrv_drained_end() can poll them.
> > > 6184 */
> > > 6185 if (qemu_get_aio_context() != old_context) {
> > > 6186 aio_context_release(old_context);
> > > 6187 }
> > >
> > > My first thought here is that given the context and the apparent
> > > expectations of where and how virtio_scsi_acquire() is used, we should
> > > probably use an independent lock on VirtIOSCSI instead of acquiring
> > > the AioContext.
> > >
> > > What do you think?
> >
> > The virtio_scsi_ctx_check() assertion seems reasonable. virtio-scsi
> > shouldn't be processing requests while AioContexts are changing.
>
> I agree.
>
> > I'm surprised that aio_disable_external() doesn't already protect this
> > code path?
>
> The conflict happens after the call to aio_enable_external():
>
> 838 static void virtio_scsi_hotunplug(HotplugHandler *hotplug_dev,
> DeviceState *dev,
> 839 Error **errp)
> 840 {
> 841 VirtIODevice *vdev = VIRTIO_DEVICE(hotplug_dev);
> 842 VirtIOSCSI *s = VIRTIO_SCSI(vdev);
> 843 SCSIDevice *sd = SCSI_DEVICE(dev);
> 844 AioContext *ctx = s->ctx ?: qemu_get_aio_context();
> 845
> 846 if (virtio_vdev_has_feature(vdev, VIRTIO_SCSI_F_HOTPLUG)) {
> 847 virtio_scsi_acquire(s);
> 848 virtio_scsi_push_event(s, sd,
> 849 VIRTIO_SCSI_T_TRANSPORT_RESET,
> 850 VIRTIO_SCSI_EVT_RESET_REMOVED);
> 851 virtio_scsi_release(s);
> 852 }
> 853
> 854 aio_disable_external(ctx);
> 855 qdev_simple_device_unplug_cb(hotplug_dev, dev, errp);
> 856 aio_enable_external(ctx);
> 857
> 858 if (s->ctx) {
> 859 virtio_scsi_acquire(s);
> 860 /* If other users keep the BlockBackend in the iothread, that's
> ok */
> 861 blk_set_aio_context(sd->conf.blk, qemu_get_aio_context(), NULL);
> ^^^^^^^^^^^^^^^^^^^
> |-> This is call that leads to releasing the context
>
> 862 virtio_scsi_release(s);
> 863 }
> 864 }
>
> I'm not sure if we can safely move the aio_enable_external() call to
> the end of the function.
I cannot think of a reason to resume virtqueue processing across lines
858-863.
Instead of calling aio_disable_external() directly, line 854 could be
changed to bdrv_drained_begin() and line 856 could be replaced with
bdrv_drained_end() on line 864.
This way in-flight requests are completed before unplug. Drained
sections include the effect of aio_disable_external() so we no longer
need to call it explicitly.
Stefan
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