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[2003:cf:d73b:4143:2bf6:228a:1b7a:e384]) by smtp.gmail.com with ESMTPSA id bi13-20020a05600c3d8d00b0040e8800fcf3sm22935620wmb.5.2024.01.23.08.40.39 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 23 Jan 2024 08:40:39 -0800 (PST) Content-Type: multipart/alternative; boundary="------------ykasGuEektmD3i4bNf0Hgwsh" Message-ID: <73e752b2-a037-4b10-a903-56fa6ad75c6e@redhat.com> Date: Tue, 23 Jan 2024 17:40:39 +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 , qemu-block@nongnu.org Cc: stefanha@redhat.com, qemu-devel@nongnu.org References: <20231221212339.164439-1-kwolf@redhat.com> <20231221212339.164439-12-kwolf@redhat.com> From: Hanna Czenczek In-Reply-To: <20231221212339.164439-12-kwolf@redhat.com> Received-SPF: pass client-ip=170.10.133.124; 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_H3=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=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 This is a multi-part message in MIME format. --------------ykasGuEektmD3i4bNf0Hgwsh Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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 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’ll look more into it. Hanna --------------ykasGuEektmD3i4bNf0Hgwsh Content-Type: text/html; charset=UTF-8 Content-Transfer-Encoding: 8bit
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’ll look more into it.

Hanna
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