From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 2F246C77B61 for ; Fri, 28 Apr 2023 07:54:47 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1psIvb-0003ed-OT; Fri, 28 Apr 2023 03:54:03 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1psIvZ-0003dx-Gm; Fri, 28 Apr 2023 03:54:01 -0400 Received: from proxmox-new.maurer-it.com ([94.136.29.106]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1psIvT-0006HD-Vz; Fri, 28 Apr 2023 03:54:01 -0400 Received: from proxmox-new.maurer-it.com (localhost.localdomain [127.0.0.1]) by proxmox-new.maurer-it.com (Proxmox) with ESMTP id 4F09A45DFE; Fri, 28 Apr 2023 09:53:53 +0200 (CEST) Message-ID: <39c503bc-fcc1-9dd9-c39e-efadc35bd97f@proxmox.com> Date: Fri, 28 Apr 2023 09:53:51 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.10.0 Subject: Re: QMP (without OOB) function running in thread different from the main thread as part of aio_poll Content-Language: en-US To: Peter Xu , Juan Quintela Cc: Kevin Wolf , Paolo Bonzini , Markus Armbruster , QEMU Developers , "open list:Block layer core" , Michael Roth , Fam Zheng , Stefan Hajnoczi , Thomas Lamprecht References: <2a61b581-5a21-c945-bb98-b6863cac0c1f@proxmox.com> <877cu7gk1g.fsf@pond.sub.org> <3ba2f8b9-9818-6601-2247-7b0e20d7ab0d@proxmox.com> <515e6a39-8515-b32b-05ce-6d7511779b1b@proxmox.com> <87zg6tbdep.fsf@secure.mitica> From: Fiona Ebner In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=94.136.29.106; envelope-from=f.ebner@proxmox.com; helo=proxmox-new.maurer-it.com X-Spam_score_int: -32 X-Spam_score: -3.3 X-Spam_bar: --- X-Spam_report: (-3.3 / 5.0 requ) BAYES_00=-1.9, NICE_REPLY_A=-1.422, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Am 27.04.23 um 16:56 schrieb Peter Xu: > On Thu, Apr 27, 2023 at 04:36:14PM +0200, Juan Quintela wrote: >> Fiona Ebner 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=) 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 ) 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 , >>>> 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 >>>> , addr=4291004224, attrs=..., >>>> buf=0x7f9029957028, len=1) at ../softmmu/physmem.c:2779 >>>> #30 0x0000558ed403ac74 in address_space_rw (as=0x558ed4ca2b20 >>>> , 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=) 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? > It is taken in the beginning, but qemu_savevm_state drops it. I suppose there is a good reason for it, because when patching out the qemu_mutex_unlock_iothread call, I get ../softmmu/cpus.c:503:qemu_mutex_lock_iothread_impl: assertion failed: (!qemu_mutex_iothread_locked())when using the snapshot-save QMP command. To avoid the assertion failure from the backtrace, I think it would be enough to ensure still having acquired the main thread's AioContext between the qemu_mutex_unlock_iothread and qemu_mutex_lock_iothread calls (which currently is not the case when snapshot_save_job_bh runs in a vCPU thread). But not sure if that is enough for full correctness. Best Regards, Fiona