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 38FC3C47258 for ; Tue, 23 Jan 2024 17:23:51 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rSKUf-0004mb-JJ; Tue, 23 Jan 2024 12:23:25 -0500 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 1rSKUe-0004mM-Bh for qemu-devel@nongnu.org; Tue, 23 Jan 2024 12:23:24 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rSKUb-0003vM-Bl for qemu-devel@nongnu.org; Tue, 23 Jan 2024 12:23:23 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1706030600; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=RwaaR3S6dqYLs7KyBvCXb7PoA39urmMNK4K1foXxwfc=; b=V2ayq80atvllMMLV9dAnNI5KEq7C08si975RxZ2GAJaMNFuSuzviQfSg0qW5WeJlvQGNEo clbUaN6lobvYw5hLrTofPyRXLQCJm4ppwOBb8m2ghZAgPN4bayGYFUW+YoPXfIcazDQ1VV OSiRV08xLPKyY47cJhDTu6kXv5Vb0Sw= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-543-9wOzXtGsOkqgNj5X-OuViQ-1; Tue, 23 Jan 2024 12:23:18 -0500 X-MC-Unique: 9wOzXtGsOkqgNj5X-OuViQ-1 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-40e9d8e9a39so39111805e9.0 for ; Tue, 23 Jan 2024 09:23:17 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1706030597; x=1706635397; h=in-reply-to:from:references:cc:to:content-language:subject :user-agent:mime-version:date:message-id:x-gm-message-state:from:to :cc:subject:date:message-id:reply-to; bh=RwaaR3S6dqYLs7KyBvCXb7PoA39urmMNK4K1foXxwfc=; b=vCf2TZU5b0PbuxHmSFDI4TX23FnL4V3EIbXxBSioNN8P02K31CUcplkZW2m0WCLPyn FLmXjxf1JeRZPNbwm9Fu0OHAusUzg8PTWdWuhyHIrgnyRLUQvp3pQMDpK1W7JaeetohV hwtCs+D2wMGiqcJY5Wj5uFTRRTBUiM7H76vtbe0m2Gl0sadUR8B1aOKpIFnR3E7JxEzR a1fTjRCsZo/NW3FWiA9cLIjd+0Q5tNZT58KDg4OpDS5wwv8UI+MT2lLjkEduWltvTBY5 lSR9fgG0zImtQgJbPYkHa+uEVqDLdSouVhhHOtS+BijwrBKDlk9XHOtBe28oobm2PkmW jh+g== X-Gm-Message-State: AOJu0YwxxzJIk99R5QHb/EZ7v80NxBU15ww8ltVZeo379c5NMRbMk9vU YWX+p4v05+vVYL5Bw27WrKO64GB41fWIadhmskqxHF5iK6Ub6ZDIhxfRbKvyRwbXCirMFMji9an XXAlLvZfVH1LoBUsd2Cvt10P60yQ5AeVeevwzXICvajZ58tJS5JuN X-Received: by 2002:a05:600c:3208:b0:40e:8903:6e1 with SMTP id r8-20020a05600c320800b0040e890306e1mr718290wmp.40.1706030597066; Tue, 23 Jan 2024 09:23:17 -0800 (PST) X-Google-Smtp-Source: AGHT+IGs75VW4/gboYCmw8wBm0XEV2OQEXspWDQgao5uwe92zxa9YfyIxjiLICYVJyAkf0+yP03CIA== X-Received: by 2002:a05:600c:3208:b0:40e:8903:6e1 with SMTP id r8-20020a05600c320800b0040e890306e1mr718283wmp.40.1706030596727; Tue, 23 Jan 2024 09:23:16 -0800 (PST) Received: from ?IPV6:2003:cf:d73b:4143:2bf6:228a:1b7a:e384? (p200300cfd73b41432bf6228a1b7ae384.dip0.t-ipconnect.de. [2003:cf:d73b:4143:2bf6:228a:1b7a:e384]) by smtp.gmail.com with ESMTPSA id iv11-20020a05600c548b00b0040d8ff79fd8sm43252595wmb.7.2024.01.23.09.23.16 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Jan 2024 09:23:16 -0800 (PST) Content-Type: multipart/alternative; boundary="------------5IwzkY0xB1bX1d7Xc0pikBAj" Message-ID: <90b2a5da-d4b5-4749-aba4-7a0d42b4f6f8@redhat.com> Date: Tue, 23 Jan 2024 18:23:15 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PULL 11/33] scsi: only access SCSIDevice->requests from one thread Content-Language: en-US To: Kevin Wolf Cc: qemu-block@nongnu.org, stefanha@redhat.com, qemu-devel@nongnu.org References: <20231221212339.164439-1-kwolf@redhat.com> <20231221212339.164439-12-kwolf@redhat.com> <73e752b2-a037-4b10-a903-56fa6ad75c6e@redhat.com> From: Hanna Czenczek In-Reply-To: Received-SPF: pass client-ip=170.10.129.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.327, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=unavailable 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 This is a multi-part message in MIME format. --------------5IwzkY0xB1bX1d7Xc0pikBAj Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 23.01.24 18:10, Kevin Wolf wrote: > Am 23.01.2024 um 17:40 hat Hanna Czenczek geschrieben: >> On 21.12.23 22:23, Kevin Wolf wrote: >>> From: Stefan Hajnoczi >>> >>> Stop depending on the AioContext lock and instead access >>> SCSIDevice->requests from only one thread at a time: >>> - When the VM is running only the BlockBackend's AioContext may access >>> the requests list. >>> - When the VM is stopped only the main loop may access the requests >>> list. >>> >>> These constraints protect the requests list without the need for locking >>> in the I/O code path. >>> >>> Note that multiple IOThreads are not supported yet because the code >>> assumes all SCSIRequests are executed from a single AioContext. Leave >>> that as future work. >>> >>> Signed-off-by: Stefan Hajnoczi >>> Reviewed-by: Eric Blake >>> Message-ID:<20231204164259.1515217-2-stefanha@redhat.com> >>> Signed-off-by: Kevin Wolf >>> --- >>> include/hw/scsi/scsi.h | 7 +- >>> hw/scsi/scsi-bus.c | 181 ++++++++++++++++++++++++++++------------- >>> 2 files changed, 131 insertions(+), 57 deletions(-) >> My reproducer forhttps://issues.redhat.com/browse/RHEL-3934 now breaks more >> often because of this commit than because of the original bug, i.e. when >> repeatedly hot-plugging and unplugging a virtio-scsi and a scsi-hd device, >> this tends to happen when unplugging the scsi-hd: >> >> {"execute":"device_del","arguments":{"id":"stg0"}} >> {"return": {}} >> qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_context: >> Assertion `ctx == blk->ctx' failed. >> >> (gdb) bt >> #0  0x00007f32a668d83c in  () at /usr/lib/libc.so.6 >> #1  0x00007f32a663d668 in raise () at /usr/lib/libc.so.6 >> #2  0x00007f32a66254b8 in abort () at /usr/lib/libc.so.6 >> #3  0x00007f32a66253dc in  () at /usr/lib/libc.so.6 >> #4  0x00007f32a6635d26 in  () at /usr/lib/libc.so.6 >> #5  0x0000556e6b4880a4 in blk_get_aio_context (blk=0x556e6e89ccf0) at >> ../block/block-backend.c:2429 >> #6  blk_get_aio_context (blk=0x556e6e89ccf0) at >> ../block/block-backend.c:2417 >> #7  0x0000556e6b112d87 in scsi_device_for_each_req_async_bh >> (opaque=0x556e6e2c6d10) at ../hw/scsi/scsi-bus.c:128 >> #8  0x0000556e6b5d1966 in aio_bh_poll (ctx=ctx@entry=0x556e6d8aa290) at >> ../util/async.c:218 >> #9  0x0000556e6b5bb16a in aio_poll (ctx=0x556e6d8aa290, >> blocking=blocking@entry=true) at ../util/aio-posix.c:722 >> #10 0x0000556e6b4564b6 in iothread_run (opaque=opaque@entry=0x556e6d89d920) >> at ../iothread.c:63 >> #11 0x0000556e6b5bde58 in qemu_thread_start (args=0x556e6d8aa9b0) at >> ../util/qemu-thread-posix.c:541 >> #12 0x00007f32a668b9eb in  () at /usr/lib/libc.so.6 >> #13 0x00007f32a670f7cc in  () at /usr/lib/libc.so.6 >> >> I don’t know anything about the problem yet, but as usual, I like >> speculation and discovering how wrong I was later on, so one thing I came >> across that’s funny about virtio-scsi is that requests can happen even while >> a disk is being attached or detached.  That is, Linux seems to probe all >> LUNs when a new virtio-scsi device is being attached, and it won’t stop just >> because a disk is being attached or removed.  So maybe that’s part of the >> problem, that we get a request while the BB is being detached, and >> temporarily in an inconsistent state (BDS context differs from BB context). > I don't know anything about the problem either, but since you already > speculated about the cause, let me speculate about the solution: > Can we hold the graph writer lock for the tran_commit() call in > bdrv_try_change_aio_context()? And of course take the reader lock for > blk_get_aio_context(), but that should be completely unproblematic. > > At the first sight I don't see a reason why this would break something, > but I've learnt not to trust my first impression with the graph locking > work... > > Of course, I also didn't check if there are more things inside of the > device emulation that need additional locking in this case, too. But > even if so, blk_get_aio_context() should never see an inconsistent > state. Ah, sorry, saw your reply only now that I hit “send”. I forgot that we do have that big lock that I thought rather to avoid :)  Sounds good and very reasonable to me.  Changing the contexts in the graph sounds like a graph change operation, and reading and comparing contexts in the graph sounds like reading the graph. Hanna --------------5IwzkY0xB1bX1d7Xc0pikBAj Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
On 23.01.24 18:10, Kevin Wolf wrote:
Am 23.01.2024 um 17:40 hat Hanna Czenczek geschrieben:

On 21.12.23 22:23, Kevin Wolf wrote:

From: Stefan Hajnoczi<stefanha@redhat.com>

Stop depending on the AioContext lock and instead access
SCSIDevice->requests from only one thread at a time:
- When the VM is running only the BlockBackend's AioContext may access
   the requests list.
- When the VM is stopped only the main loop may access the requests
   list.

These constraints protect the requests list without the need for locking
in the I/O code path.

Note that multiple IOThreads are not supported yet because the code
assumes all SCSIRequests are executed from a single AioContext. Leave
that as future work.

Signed-off-by: Stefan Hajnoczi<stefanha@redhat.com>
Reviewed-by: Eric Blake<eblake@redhat.com>
Message-ID:<20231204164259.1515217-2-stefanha@redhat.com>
Signed-off-by: Kevin Wolf<kwolf@redhat.com>
---
  include/hw/scsi/scsi.h |   7 +-
  hw/scsi/scsi-bus.c     | 181 ++++++++++++++++++++++++++++-------------
  2 files changed, 131 insertions(+), 57 deletions(-)

My reproducer for https://issues.redhat.com/browse/RHEL-3934 now breaks more
often because of this commit than because of the original bug, i.e. when
repeatedly hot-plugging and unplugging a virtio-scsi and a scsi-hd device,
this tends to happen when unplugging the scsi-hd:

{"execute":"device_del","arguments":{"id":"stg0"}}
{"return": {}}
qemu-system-x86_64: ../block/block-backend.c:2429: blk_get_aio_context:
Assertion `ctx == blk->ctx' failed.

(gdb) bt
#0  0x00007f32a668d83c in  () at /usr/lib/libc.so.6
#1  0x00007f32a663d668 in raise () at /usr/lib/libc.so.6
#2  0x00007f32a66254b8 in abort () at /usr/lib/libc.so.6
#3  0x00007f32a66253dc in  () at /usr/lib/libc.so.6
#4  0x00007f32a6635d26 in  () at /usr/lib/libc.so.6
#5  0x0000556e6b4880a4 in blk_get_aio_context (blk=0x556e6e89ccf0) at
../block/block-backend.c:2429
#6  blk_get_aio_context (blk=0x556e6e89ccf0) at
../block/block-backend.c:2417
#7  0x0000556e6b112d87 in scsi_device_for_each_req_async_bh
(opaque=0x556e6e2c6d10) at ../hw/scsi/scsi-bus.c:128
#8  0x0000556e6b5d1966 in aio_bh_poll (ctx=ctx@entry=0x556e6d8aa290) at
../util/async.c:218
#9  0x0000556e6b5bb16a in aio_poll (ctx=0x556e6d8aa290,
blocking=blocking@entry=true) at ../util/aio-posix.c:722
#10 0x0000556e6b4564b6 in iothread_run (opaque=opaque@entry=0x556e6d89d920)
at ../iothread.c:63
#11 0x0000556e6b5bde58 in qemu_thread_start (args=0x556e6d8aa9b0) at
../util/qemu-thread-posix.c:541
#12 0x00007f32a668b9eb in  () at /usr/lib/libc.so.6
#13 0x00007f32a670f7cc in  () at /usr/lib/libc.so.6

I don’t know anything about the problem yet, but as usual, I like
speculation and discovering how wrong I was later on, so one thing I came
across that’s funny about virtio-scsi is that requests can happen even while
a disk is being attached or detached.  That is, Linux seems to probe all
LUNs when a new virtio-scsi device is being attached, and it won’t stop just
because a disk is being attached or removed.  So maybe that’s part of the
problem, that we get a request while the BB is being detached, and
temporarily in an inconsistent state (BDS context differs from BB context).

I don't know anything about the problem either, but since you already
speculated about the cause, let me speculate about the solution:
Can we hold the graph writer lock for the tran_commit() call in
bdrv_try_change_aio_context()? And of course take the reader lock for
blk_get_aio_context(), but that should be completely unproblematic.

At the first sight I don't see a reason why this would break something,
but I've learnt not to trust my first impression with the graph locking
work...

Of course, I also didn't check if there are more things inside of the
device emulation that need additional locking in this case, too. But
even if so, blk_get_aio_context() should never see an inconsistent
state.

Ah, sorry, saw your reply only now that I hit “send”.

I forgot that we do have that big lock that I thought rather to avoid :)  Sounds good and very reasonable to me.  Changing the contexts in the graph sounds like a graph change operation, and reading and comparing contexts in the graph sounds like reading the graph.

Hanna
--------------5IwzkY0xB1bX1d7Xc0pikBAj--