Re: [PATCH v4 04/20] virtio-scsi: stop using aio_disable_external() during unplug

[Kevin Wolf <kwolf@redhat.com>]

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Am 01.05.2023 um 17:09 hat Stefan Hajnoczi geschrieben:
> On Fri, Apr 28, 2023 at 04:22:55PM +0200, Kevin Wolf wrote:
> > Am 25.04.2023 um 19:27 hat Stefan Hajnoczi geschrieben:
> > > This patch is part of an effort to remove the aio_disable_external()
> > > API because it does not fit in a multi-queue block layer world where
> > > many AioContexts may be submitting requests to the same disk.
> > > 
> > > The SCSI emulation code is already in good shape to stop using
> > > aio_disable_external(). It was only used by commit 9c5aad84da1c
> > > ("virtio-scsi: fixed virtio_scsi_ctx_check failed when detaching scsi
> > > disk") to ensure that virtio_scsi_hotunplug() works while the guest
> > > driver is submitting I/O.
> > > 
> > > Ensure virtio_scsi_hotunplug() is safe as follows:
> > > 
> > > 1. qdev_simple_device_unplug_cb() -> qdev_unrealize() ->
> > >    device_set_realized() calls qatomic_set(&dev->realized, false) so
> > >    that future scsi_device_get() calls return NULL because they exclude
> > >    SCSIDevices with realized=false.
> > > 
> > >    That means virtio-scsi will reject new I/O requests to this
> > >    SCSIDevice with VIRTIO_SCSI_S_BAD_TARGET even while
> > >    virtio_scsi_hotunplug() is still executing. We are protected against
> > >    new requests!
> > > 
> > > 2. Add a call to scsi_device_purge_requests() from scsi_unrealize() so
> > >    that in-flight requests are cancelled synchronously. This ensures
> > >    that no in-flight requests remain once qdev_simple_device_unplug_cb()
> > >    returns.
> > > 
> > > Thanks to these two conditions we don't need aio_disable_external()
> > > anymore.
> > > 
> > > Cc: Zhengui Li <lizhengui@huawei.com>
> > > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> > > Reviewed-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > 
> > qemu-iotests 040 starts failing for me after this patch, with what looks
> > like a use-after-free error of some kind.
> > 
> > (gdb) bt
> > #0  0x000055b6e3e1f31c in job_type (job=0xe3e3e3e3e3e3e3e3) at ../job.c:238
> > #1  0x000055b6e3e1cee5 in is_block_job (job=0xe3e3e3e3e3e3e3e3) at ../blockjob.c:41
> > #2  0x000055b6e3e1ce7d in block_job_next_locked (bjob=0x55b6e72b7570) at ../blockjob.c:54
> > #3  0x000055b6e3df6370 in blockdev_mark_auto_del (blk=0x55b6e74af0a0) at ../blockdev.c:157
> > #4  0x000055b6e393e23b in scsi_qdev_unrealize (qdev=0x55b6e7c04d40) at ../hw/scsi/scsi-bus.c:303
> > #5  0x000055b6e3db0d0e in device_set_realized (obj=0x55b6e7c04d40, value=false, errp=0x55b6e497c918 <error_abort>) at ../hw/core/qdev.c:599
> > #6  0x000055b6e3dba36e in property_set_bool (obj=0x55b6e7c04d40, v=0x55b6e7d7f290, name=0x55b6e41bd6d8 "realized", opaque=0x55b6e7246d20, errp=0x55b6e497c918 <error_abort>)
> >     at ../qom/object.c:2285
> > #7  0x000055b6e3db7e65 in object_property_set (obj=0x55b6e7c04d40, name=0x55b6e41bd6d8 "realized", v=0x55b6e7d7f290, errp=0x55b6e497c918 <error_abort>) at ../qom/object.c:1420
> > #8  0x000055b6e3dbd84a in object_property_set_qobject (obj=0x55b6e7c04d40, name=0x55b6e41bd6d8 "realized", value=0x55b6e74c1890, errp=0x55b6e497c918 <error_abort>)
> >     at ../qom/qom-qobject.c:28
> > #9  0x000055b6e3db8570 in object_property_set_bool (obj=0x55b6e7c04d40, name=0x55b6e41bd6d8 "realized", value=false, errp=0x55b6e497c918 <error_abort>) at ../qom/object.c:1489
> > #10 0x000055b6e3daf2b5 in qdev_unrealize (dev=0x55b6e7c04d40) at ../hw/core/qdev.c:306
> > #11 0x000055b6e3db509d in qdev_simple_device_unplug_cb (hotplug_dev=0x55b6e81c3630, dev=0x55b6e7c04d40, errp=0x7ffec5519200) at ../hw/core/qdev-hotplug.c:72
> > #12 0x000055b6e3c520f9 in virtio_scsi_hotunplug (hotplug_dev=0x55b6e81c3630, dev=0x55b6e7c04d40, errp=0x7ffec5519200) at ../hw/scsi/virtio-scsi.c:1065
> > #13 0x000055b6e3db4dec in hotplug_handler_unplug (plug_handler=0x55b6e81c3630, plugged_dev=0x55b6e7c04d40, errp=0x7ffec5519200) at ../hw/core/hotplug.c:56
> > #14 0x000055b6e3a28f84 in qdev_unplug (dev=0x55b6e7c04d40, errp=0x7ffec55192e0) at ../softmmu/qdev-monitor.c:935
> > #15 0x000055b6e3a290fa in qmp_device_del (id=0x55b6e74c1760 "scsi0", errp=0x7ffec55192e0) at ../softmmu/qdev-monitor.c:955
> > #16 0x000055b6e3fb0a5f in qmp_marshal_device_del (args=0x7f61cc005eb0, ret=0x7f61d5a8ae38, errp=0x7f61d5a8ae40) at qapi/qapi-commands-qdev.c:114
> > #17 0x000055b6e3fd52e1 in do_qmp_dispatch_bh (opaque=0x7f61d5a8ae08) at ../qapi/qmp-dispatch.c:128
> > #18 0x000055b6e4007b9e in aio_bh_call (bh=0x55b6e7dea730) at ../util/async.c:155
> > #19 0x000055b6e4007d2e in aio_bh_poll (ctx=0x55b6e72447c0) at ../util/async.c:184
> > #20 0x000055b6e3fe3b45 in aio_dispatch (ctx=0x55b6e72447c0) at ../util/aio-posix.c:421
> > #21 0x000055b6e4009544 in aio_ctx_dispatch (source=0x55b6e72447c0, callback=0x0, user_data=0x0) at ../util/async.c:326
> > #22 0x00007f61ddc14c7f in g_main_dispatch (context=0x55b6e7244b20) at ../glib/gmain.c:3454
> > #23 g_main_context_dispatch (context=0x55b6e7244b20) at ../glib/gmain.c:4172
> > #24 0x000055b6e400a7e8 in glib_pollfds_poll () at ../util/main-loop.c:290
> > #25 0x000055b6e400a0c2 in os_host_main_loop_wait (timeout=0) at ../util/main-loop.c:313
> > #26 0x000055b6e4009fa2 in main_loop_wait (nonblocking=0) at ../util/main-loop.c:592
> > #27 0x000055b6e3a3047b in qemu_main_loop () at ../softmmu/runstate.c:731
> > #28 0x000055b6e3dab27d in qemu_default_main () at ../softmmu/main.c:37
> > #29 0x000055b6e3dab2b8 in main (argc=24, argv=0x7ffec55196a8) at ../softmmu/main.c:48
> > (gdb) p jobs
> > $4 = {lh_first = 0x0}
> 
> I wasn't able to reproduce this with gcc 13.1.1 or clang 16.0.1:
> 
>   $ tests/qemu-iotests/check -qcow2 040
> 
> Any suggestions on how to reproduce the issue?

It happens consistently for me with the same command line, both with gcc
and clang.

gcc (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4)
clang version 15.0.7 (Fedora 15.0.7-2.fc37)

Maybe there is a semantic merge conflict? I have applied the series on
top of master (05d50ba2d4) and my block branch (88f81f7bc8).

Kevin

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