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Wed, 13 May 2020 03:20:51 +0000 (UTC) Subject: Re: [PATCH v2 5/5] vhost: add device started check in migration set log To: Dima Stepanov References: <20200511092541.GA27558@dimastep-nix> <20200512093521.GA5363@dimastep-nix> From: Jason Wang Message-ID: <5afd8f14-ed9f-d3e1-3e7f-649a5140db28@redhat.com> Date: Wed, 13 May 2020 11:20:50 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.7.0 MIME-Version: 1.0 In-Reply-To: <20200512093521.GA5363@dimastep-nix> Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.13 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=207.211.31.81; envelope-from=jasowang@redhat.com; helo=us-smtp-delivery-1.mimecast.com X-detected-operating-system: by eggs.gnu.org: First seen = 2020/05/12 21:16:54 X-ACL-Warn: Detected OS = Linux 2.2.x-3.x [generic] [fuzzy] X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=0.001, 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_H2=-0.001, SPF_PASS=-0.001, URIBL_BLOCKED=0.001 autolearn=_AUTOLEARN X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: fam@euphon.net, kwolf@redhat.com, stefanha@redhat.com, qemu-block@nongnu.org, mst@redhat.com, qemu-devel@nongnu.org, dgilbert@redhat.com, arei.gonglei@huawei.com, fengli@smartx.com, yc-core@yandex-team.ru, pbonzini@redhat.com, marcandre.lureau@redhat.com, raphael.norwitz@nutanix.com, mreitz@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 2020/5/12 下午5:35, Dima Stepanov wrote: > On Tue, May 12, 2020 at 11:32:50AM +0800, Jason Wang wrote: >> On 2020/5/11 下午5:25, Dima Stepanov wrote: >>> On Mon, May 11, 2020 at 11:15:53AM +0800, Jason Wang wrote: >>>> On 2020/4/30 下午9:36, Dima Stepanov wrote: >>>>> If vhost-user daemon is used as a backend for the vhost device, then we >>>>> should consider a possibility of disconnect at any moment. If such >>>>> disconnect happened in the vhost_migration_log() routine the vhost >>>>> device structure will be clean up. >>>>> At the start of the vhost_migration_log() function there is a check: >>>>> if (!dev->started) { >>>>> dev->log_enabled = enable; >>>>> return 0; >>>>> } >>>>> To be consistent with this check add the same check after calling the >>>>> vhost_dev_set_log() routine. This in general help not to break a >>>>> migration due the assert() message. But it looks like that this code >>>>> should be revised to handle these errors more carefully. >>>>> >>>>> In case of vhost-user device backend the fail paths should consider the >>>>> state of the device. In this case we should skip some function calls >>>>> during rollback on the error paths, so not to get the NULL dereference >>>>> errors. >>>>> >>>>> Signed-off-by: Dima Stepanov >>>>> --- >>>>> hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++---- >>>>> 1 file changed, 35 insertions(+), 4 deletions(-) >>>>> >>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c >>>>> index 3ee50c4..d5ab96d 100644 >>>>> --- a/hw/virtio/vhost.c >>>>> +++ b/hw/virtio/vhost.c >>>>> @@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev *dev, >>>>> static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log) >>>>> { >>>>> int r, i, idx; >>>>> + >>>>> + if (!dev->started) { >>>>> + /* >>>>> + * If vhost-user daemon is used as a backend for the >>>>> + * device and the connection is broken, then the vhost_dev >>>>> + * structure will be reset all its values to 0. >>>>> + * Add additional check for the device state. >>>>> + */ >>>>> + return -1; >>>>> + } >>>>> + >>>>> r = vhost_dev_set_features(dev, enable_log); >>>>> if (r < 0) { >>>>> goto err_features; >>>>> @@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log) >>>>> } >>>>> return 0; >>>>> err_vq: >>>>> - for (; i >= 0; --i) { >>>>> + /* >>>>> + * Disconnect with the vhost-user daemon can lead to the >>>>> + * vhost_dev_cleanup() call which will clean up vhost_dev >>>>> + * structure. >>>>> + */ >>>>> + for (; dev->started && (i >= 0); --i) { >>>>> idx = dev->vhost_ops->vhost_get_vq_index( >>>> Why need the check of dev->started here, can started be modified outside >>>> mainloop? If yes, I don't get the check of !dev->started in the beginning of >>>> this function. >>>> >>> No dev->started can't change outside the mainloop. The main problem is >>> only for the vhost_user_blk daemon. Consider the case when we >>> successfully pass the dev->started check at the beginning of the >>> function, but after it we hit the disconnect on the next call on the >>> second or third iteration: >>> r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx, enable_log); >>> The unix socket backend device will call the disconnect routine for this >>> device and reset the structure. So the structure will be reset (and >>> dev->started set to false) inside this set_addr() call. >> I still don't get here. I think the disconnect can not happen in the middle >> of vhost_dev_set_log() since both of them were running in mainloop. And even >> if it can, we probably need other synchronization mechanism other than >> simple check here. > Disconnect isn't happened in the separate thread it is happened in this > routine inside vhost_dev_set_log. When for instance vhost_user_write() > call failed: > vhost_user_set_log_base() > vhost_user_write() > vhost_user_blk_disconnect() > vhost_dev_cleanup() > vhost_user_backend_cleanup() > So the point is that if we somehow got a disconnect with the > vhost-user-blk daemon before the vhost_user_write() call then it will > continue clean up by running vhost_user_blk_disconnect() function. I > wrote a more detailed backtrace stack in the separate thread, which is > pretty similar to what we have here: > Re: [PATCH v2 4/5] vhost: check vring address before calling unmap > The places are different but the problem is pretty similar. Yes. > > So if vhost-user commands handshake then everything is fine and > reconnect will work as expected. The only problem is how to handle > reconnect properly between vhost-user command send/receive. > > As i wrote we have a test: > - run src VM with vhost-usr-blk daemon used > - run fio inside it > - perform reconnect every X seconds (just kill and restart daemon), > X is random > - run dst VM > - perform migration > - fio should complete in dst VM > And we cycle this test like forever. > So it fails once per ~25 iteration. By adding some delays inside qemu we > were able to make the race window larger. It would be better if we can draft some qtest for this. Thanks >