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[2003:cf:d73b:4169:9eef:dd9f:57fb:1235]) by smtp.gmail.com with ESMTPSA id cm2-20020a170907938200b00a2cc5199dd0sm1238341ejc.135.2024.01.25.09.32.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Jan 2024 09:32:47 -0800 (PST) Message-ID: <08a66849-f190-4756-9b01-666f0d66afb6@redhat.com> Date: Thu, 25 Jan 2024 18:32:46 +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: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit 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: -35 X-Spam_score: -3.6 X-Spam_bar: --- X-Spam_report: (-3.6 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.5, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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=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 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 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. [...] >> 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. Actually, now that completely unproblematic part is giving me trouble.  I wanted to just put a graph lock into blk_get_aio_context() (making it a coroutine with a wrapper), but callers of blk_get_aio_context() generally assume the context is going to stay the BB’s context for as long as their AioContext * variable is in scope.  I was tempted to think callers know what happens to the BB they pass to blk_get_aio_context(), and it won’t change contexts so easily, but then I remembered this is exactly what happens in this case; we run scsi_device_for_each_req_async_bh() in one thread (which calls blk_get_aio_context()), and in the other, we change the BB’s context. It seems like there are very few blk_* functions right now that require taking a graph lock around it, so I’m hesitant to go that route.  But if we’re protecting a BB’s context via the graph write lock, I can’t think of a way around having to take a read lock whenever reading a BB’s context, and holding it for as long as we assume that context to remain the BB’s context.  It’s also hard to figure out how long that is, case by case; for example, dma_blk_read() schedules an AIO function in the BB context; but we probably don’t care that this context remains the BB’s context until the request is done.  In the case of scsi_device_for_each_req_async_bh(), we already take care to re-schedule it when it turns out the context is outdated, so it does seem quite important here, and we probably want to keep the lock until after the QTAILQ_FOREACH_SAFE() loop. On a tangent, this TOCTTOU problem makes me wary of other blk_* functions that query information.  For example, fuse_read() (in block/export/fuse.c) truncates requests to the BB length.  But what if the BB length changes concurrently between blk_getlength() and blk_pread()?  While we can justify using the graph lock for a BB’s AioContext, we can’t use it for other metadata like its length. Hanna