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[2003:cf:d73b:41d9:5581:da3:1344:320c]) by smtp.gmail.com with ESMTPSA id f24-20020a193818000000b005100dc5faf8sm239582lfa.178.2024.01.24.04.12.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 24 Jan 2024 04:12:49 -0800 (PST) Content-Type: multipart/alternative; boundary="------------5G0XJLuenLrKDGzI7Nj9AIcu" Message-ID: Date: Wed, 24 Jan 2024 13:12:47 +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=170.10.133.124; envelope-from=hreitz@redhat.com; helo=us-smtp-delivery-124.mimecast.com 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. --------------5G0XJLuenLrKDGzI7Nj9AIcu 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. [...] > 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. I tried this, and it’s not easy taking the lock just for tran_commit(), because some callers of bdrv_try_change_aio_context() already hold the write lock (specifically bdrv_attach_child_common(), bdrv_attach_child_common_abort(), and bdrv_root_unref_child()[1]), and qmp_x_blockdev_set_iothread() holds the read lock.  Other callers don’t hold any lock[2]. So I’m not sure whether we should mark all of bdrv_try_change_aio_context() as GRAPH_WRLOCK and then make all callers take the lock, or really only take it for tran_commit(), and have callers release the lock around bdrv_try_change_aio_context(). Former sounds better to naïve me. (In any case, FWIW, having blk_set_aio_context() take the write lock, and scsi_device_for_each_req_async_bh() take the read lock[3], does fix the assertion failure.) Hanna [1] bdrv_root_unref_child() is not marked as GRAPH_WRLOCK, but it’s callers generally seem to ensure that the lock is taken when calling it. [2] blk_set_aio_context() (evidently), blk_exp_add(), external_snapshot_abort(), {blockdev,drive}_backup_action(), qmp_{blockdev,drive}_mirror() [3] I’ve made the _bh a coroutine (for bdrv_graph_co_rdlock()) and replaced the aio_bh_schedule_oneshot() by aio_co_enter() – hope that’s right. --------------5G0XJLuenLrKDGzI7Nj9AIcu 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.

[...]

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.

I tried this, and it’s not easy taking the lock just for tran_commit(), because some callers of bdrv_try_change_aio_context() already hold the write lock (specifically bdrv_attach_child_common(), bdrv_attach_child_common_abort(), and bdrv_root_unref_child()[1]), and qmp_x_blockdev_set_iothread() holds the read lock.  Other callers don’t hold any lock[2].

So I’m not sure whether we should mark all of bdrv_try_change_aio_context() as GRAPH_WRLOCK and then make all callers take the lock, or really only take it for tran_commit(), and have callers release the lock around bdrv_try_change_aio_context().  Former sounds better to naïve me.

(In any case, FWIW, having blk_set_aio_context() take the write lock, and scsi_device_for_each_req_async_bh() take the read lock[3], does fix the assertion failure.)

Hanna

[1] bdrv_root_unref_child() is not marked as GRAPH_WRLOCK, but it’s callers generally seem to ensure that the lock is taken when calling it.

[2] blk_set_aio_context() (evidently), blk_exp_add(), external_snapshot_abort(), {blockdev,drive}_backup_action(), qmp_{blockdev,drive}_mirror()

[3] I’ve made the _bh a coroutine (for bdrv_graph_co_rdlock()) and replaced the aio_bh_schedule_oneshot() by aio_co_enter() – hope that’s right.
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