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

[Kevin Wolf <kwolf@xxxxxxxxxx>]

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@xxxxxxxxxx>
> Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Reviewed-by: Daniil Tatianin <d-tatianin@xxxxxxxxxxxxxx>
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>

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}

Kevin