Re: QMP (without OOB) function running in thread different from the main thread as part of aio_poll

[Peter Xu]

On Thu, Apr 27, 2023 at 04:36:14PM +0200, Juan Quintela wrote:
> Fiona Ebner <f.ebner@proxmox.com> wrote:
> > Am 27.04.23 um 13:03 schrieb Kevin Wolf:
> >> Am 26.04.2023 um 16:31 hat Fiona Ebner geschrieben:
> >>> Am 20.04.23 um 08:55 schrieb Paolo Bonzini:
> 
> Hi
> 
> > Our function is a custom variant of saving a snapshot and uses
> > qemu_savevm_state_setup(), which is why the qemu_mutex_unlock_iothread()
> > is there. I looked for inspiration for how upstream does things and it
> > turns out that upstream QEMU v8.0.0 has essentially the same issue with
> > snapshot-save. When snapshot_save_job_bh runs in a vCPU thread instead
> > of the main thread, the situation is the same: after
> > qemu_mutex_unlock_iothread(), qemu_get_current_aio_context() will return
> > 0x0 and then the assertion in the AIO_WAIT_WHILE_INTERNAL macro fails
> > (this time the generated coroutine wrapper is bdrv_writev_vmstate)[0].
> >
> >
> > So all bottom halves scheduled for the main thread's AioContext can
> > potentially get to run in a vCPU thread and need to be very careful with
> > things like qemu_mutex_unlock_iothread.
> >
> > Is qemu_get_current_aio_context() returning 0x0 expected? I haven't
> > looked into why it happens yet. Does there need to be a way to drop the
> > BQL without also giving up the main thread's AioContext or would it be
> > enough to re-acquire the context?
> >
> > CC-ing Juan as the migration maintainer.
> 
> This is the world backwards.
> The tradition is that migration people blame block layer people for
> breaking things and for help, not the other way around O:-)
> 
> > Best Regards,
> > Fiona
> >
> > [0]:
> >> Thread 21 "CPU 0/KVM" received signal SIGABRT, Aborted.
> >> __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
> >> 50 ../sysdeps/unix/sysv/linux/raise.c: No such file or directory.
> >> (gdb) bt
> >> #0  __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:50
> >> #1  0x00007f9027b3e537 in __GI_abort () at abort.c:79
> >> #2 0x00007f9027b3e40f in __assert_fail_base (fmt=0x7f9027cb66a8
> >> "%s%s%s:%u: %s%sAssertion `%s' failed.\n%n",
> >> assertion=0x558ed44fcec0 "qemu_get_current_aio_context() ==
> >> qemu_get_aio_context()", file=0x558ed44fce80
> >> "/home/febner/repos/qemu/block/block-gen.h", line=43,
> >> function=<optimized out>) at assert.c:92
> >> #3 0x00007f9027b4d662 in __GI___assert_fail
> >> (assertion=0x558ed44fcec0 "qemu_get_current_aio_context() ==
> >> qemu_get_aio_context()", file=0x558ed44fce80
> >> "/home/febner/repos/qemu/block/block-gen.h", line=43,
> >> function=0x558ed44fcf80 <__PRETTY_FUNCTION__.14> "bdrv_poll_co") at
> >> assert.c:101
> >> #4  0x0000558ed412df5f in bdrv_poll_co (s=0x7f8ffcff37a0) at 
> >> /home/febner/repos/qemu/block/block-gen.h:43
> >> #5  0x0000558ed412f4cd in bdrv_writev_vmstate (bs=0x558ed60536a0, 
> >> qiov=0x7f8ffcff3840, pos=0) at block/block-gen.c:809
> >> #6 0x0000558ed3df36d0 in qio_channel_block_writev
> >> (ioc=0x7f8ff40ac060, iov=0x7f8ff43f6350, niov=1, fds=0x0, nfds=0,
> >> flags=0, errp=0x7f8ffcff39c0) at ../migration/channel-block.c:89
> >> #7 0x0000558ed40feedb in qio_channel_writev_full
> >> (ioc=0x7f8ff40ac060, iov=0x7f8ff43f6350, niov=1, fds=0x0, nfds=0,
> >> flags=0, errp=0x7f8ffcff39c0) at ../io/channel.c:108
> >> #8 0x0000558ed40ff3c3 in qio_channel_writev_full_all
> >> (ioc=0x7f8ff40ac060, iov=0x7f8ff4648040, niov=1, fds=0x0, nfds=0,
> >> flags=0, errp=0x7f8ffcff39c0) at ../io/channel.c:263
> >> #9 0x0000558ed40ff2e4 in qio_channel_writev_all (ioc=0x7f8ff40ac060,
> >> iov=0x7f8ff4648040, niov=1, errp=0x7f8ffcff39c0) at
> >> ../io/channel.c:242
> >> #10 0x0000558ed3dee4dc in qemu_fflush (f=0x7f8ff4640000) at 
> >> ../migration/qemu-file.c:302
> >> #11 0x0000558ed4050f91 in ram_save_setup (f=0x7f8ff4640000, 
> >> opaque=0x558ed4ca34c0 <ram_state>) at ../migration/ram.c:3302
> >> #12 0x0000558ed3e141c8 in qemu_savevm_state_setup (f=0x7f8ff4640000) at 
> >> ../migration/savevm.c:1266
> >> #13 0x0000558ed3e14eed in qemu_savevm_state (f=0x7f8ff4640000, 
> >> errp=0x558ed68c5238) at ../migration/savevm.c:1626
> >> #14 0x0000558ed3e1755e in save_snapshot (name=0x558ed72af790
> >> "snap0", overwrite=false, vmstate=0x558ed6708ce0 "scsi0",
> >> has_devices=true, devices=0x558ed66d6a60, errp=0x558ed68c5238) at
> >> ../migration/savevm.c:2954
> >> #15 0x0000558ed3e17fb1 in snapshot_save_job_bh (opaque=0x558ed68c5170) at 
> >> ../migration/savevm.c:3253
> >> #16 0x0000558ed42f050a in aio_bh_call (bh=0x558ed671ae00) at 
> >> ../util/async.c:155
> >> #17 0x0000558ed42f0615 in aio_bh_poll (ctx=0x558ed5c62910) at 
> >> ../util/async.c:184
> >> #18 0x0000558ed42d47b8 in aio_poll (ctx=0x558ed5c62910, blocking=true) at 
> >> ../util/aio-posix.c:721
> >> #19 0x0000558ed412df1c in bdrv_poll_co (s=0x7f8ffcff3eb0) at 
> >> /home/febner/repos/qemu/block/block-gen.h:43
> >> #20 0x0000558ed4130c3a in blk_pwrite (blk=0x558ed5ed4f60,
> >> offset=230912, bytes=512, buf=0x7f8ffc438600, flags=0) at
> >> block/block-gen.c:1650
> >> #21 0x0000558ed3ba9078 in pflash_update (pfl=0x558ed5eb7b30, 
> >> offset=230912, size=1) at ../hw/block/pflash_cfi01.c:394
> >> #22 0x0000558ed3ba9749 in pflash_write (pfl=0x558ed5eb7b30,
> >> offset=231232, value=0, width=1, be=0) at
> >> ../hw/block/pflash_cfi01.c:522
> >> #23 0x0000558ed3ba9cda in pflash_mem_write_with_attrs
> >> (opaque=0x558ed5eb7b30, addr=231232, value=0, len=1, attrs=...) at
> >> ../hw/block/pflash_cfi01.c:681
> >> #24 0x0000558ed402a36a in memory_region_write_with_attrs_accessor
> >> (mr=0x558ed5eb7ef0, addr=231232, value=0x7f8ffcff40c8, size=1,
> >> shift=0, mask=255, attrs=...) at ../softmmu/memory.c:514
> >> #25 0x0000558ed402a4a9 in access_with_adjusted_size (addr=231232,
> >> value=0x7f8ffcff40c8, size=1, access_size_min=1, access_size_max=4,
> >> access_fn=0x558ed402a270 <memory_region_write_with_attrs_accessor>,
> >> mr=0x558ed5eb7ef0, attrs=...) at ../softmmu/memory.c:555
> >> #26 0x0000558ed402d5de in memory_region_dispatch_write
> >> (mr=0x558ed5eb7ef0, addr=231232, data=0, op=MO_8, attrs=...) at
> >> ../softmmu/memory.c:1522
> >> #27 0x0000558ed403a6f4 in flatview_write_continue
> >> (fv=0x558ed66d62c0, addr=4291004224, attrs=..., ptr=0x7f9029957028,
> >> len=1, addr1=231232, l=1, mr=0x558ed5eb7ef0) at
> >> ../softmmu/physmem.c:2641
> >> #28 0x0000558ed403a857 in flatview_write (fv=0x558ed66d62c0,
> >> addr=4291004224, attrs=..., buf=0x7f9029957028, len=1) at
> >> ../softmmu/physmem.c:2683
> >> #29 0x0000558ed403ac07 in address_space_write (as=0x558ed4ca2b20
> >> <address_space_memory>, addr=4291004224, attrs=...,
> >> buf=0x7f9029957028, len=1) at ../softmmu/physmem.c:2779
> >> #30 0x0000558ed403ac74 in address_space_rw (as=0x558ed4ca2b20
> >> <address_space_memory>, addr=4291004224, attrs=...,
> >> buf=0x7f9029957028, len=1, is_write=true) at
> >> ../softmmu/physmem.c:2789
> >> #31 0x0000558ed40cea88 in kvm_cpu_exec (cpu=0x558ed622a910) at 
> >> ../accel/kvm/kvm-all.c:2989
> >> #32 0x0000558ed40d179a in kvm_vcpu_thread_fn (arg=0x558ed622a910) at 
> >> ../accel/kvm/kvm-accel-ops.c:51
> >> #33 0x0000558ed42d925f in qemu_thread_start (args=0x558ed5c68c80) at 
> >> ../util/qemu-thread-posix.c:541
> >> #34 0x00007f9028ab7ea7 in start_thread (arg=<optimized out>) at 
> >> pthread_create.c:477
> >> #35 0x00007f9027c18a2f in clone () at 
> >> ../sysdeps/unix/sysv/linux/x86_64/clone.S:95

Totally unfamiliar with block jobs, but... does it mean that
snapshot_*_job_bh()s should just always make sure BQL taken?

Thanks,

-- 
Peter Xu