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[82.53.135.65]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-49541e9ee07sm70836725e9.15.2026.07.16.09.13.06 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 16 Jul 2026 09:13:07 -0700 (PDT) Date: Thu, 16 Jul 2026 18:13:02 +0200 From: Stefano Garzarella To: Andrey Drobyshev Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, mst@redhat.com, stefanha@redhat.com, dongli.zhang@oracle.com, maciej.szmigiero@oracle.com, bchaney@akamai.com, mark.kanda@oracle.com, ptikhomirov@virtuozzo.com, den@openvz.org Subject: Re: [PATCH v4 4/5] vhost: synchronize with RCU readers when freeing workers Message-ID: References: <20260714151638.143019-1-andrey.drobyshev@virtuozzo.com> <20260714151638.143019-5-andrey.drobyshev@virtuozzo.com> <2f680236-f4c1-418b-8401-4dea1230caf0@virtuozzo.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii; format=flowed Content-Disposition: inline In-Reply-To: <2f680236-f4c1-418b-8401-4dea1230caf0@virtuozzo.com> On Thu, Jul 16, 2026 at 06:39:48PM +0300, Andrey Drobyshev wrote: >On 7/16/26 11:57 AM, Stefano Garzarella wrote: >> On Tue, Jul 14, 2026 at 06:16:37PM +0300, Andrey Drobyshev wrote: >>> vhost_vq_work_queue() only holds the RCU read lock while it dereferences >>> vq->worker and queues work on it. vhost_workers_free() however clears >>> the vq->worker pointers and immediately frees the workers, without >>> waiting for a grace period. A caller that fetched the worker right >>> before the pointer was cleared can therefore still be queueing work on >>> it while it is freed. And even when the queueing itself wins the race, >>> the work is never run, so its VHOST_WORK_QUEUED bit stays set and all >>> future attempts to queue it are silently skipped. >>> >>> None of the current callers can actually hit this: net and scsi stop >>> their virtqueues before the workers are freed, and vsock unhashes the >>> device and does synchronize_rcu() of its own in vhost_vsock_dev_release() >>> before the workers go away. But the upcoming VHOST_RESET_OWNER support >>> in vhost-vsock keeps the device hashed while its workers are freed, so >>> the lockless send/cancel paths become able to race with the teardown. >>> >>> Close this the way vhost_worker_killed() already does: clear the >>> vq->worker pointers, wait for a grace period, run whatever the last >>> readers may have queued, and only then free the workers. The >>> synchronize_rcu() is skipped if the device has no workers, so cleanup of >>> devices which never got an owner stays cheap. >>> >> >> Do we need a Fixes tag for this? >> > >I'm guessing it should be: > >Fixes: 228a27cf78af ("vhost: Allow worker switching while work is queueing") > >> Thanks for pointing out that the issue wasn't occurring, but I think we >> should add it because it's a sneaky problem we discovered by chance. >> IMO the code should already have `synchronize_rcu()` after >> `rcu_assign_pointer()` loop. >> >> @Michael, what do you think? >> >>> Suggested-by: Stefano Garzarella >>> Signed-off-by: Andrey Drobyshev >>> --- >>> drivers/vhost/vhost.c | 15 +++++++++++++++ >>> 1 file changed, 15 insertions(+) >>> >>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c >>> index 4c525b3e16ea..0d1414d40f4e 100644 >>> --- a/drivers/vhost/vhost.c >>> +++ b/drivers/vhost/vhost.c >>> @@ -729,6 +729,21 @@ static void vhost_workers_free(struct vhost_dev *dev) >>> >>> for (i = 0; i < dev->nvqs; i++) >>> rcu_assign_pointer(dev->vqs[i]->worker, NULL); >>> + >>> + /* >>> + * vhost_vq_work_queue() reads vq->worker under rcu_read_lock(), so a >>> + * caller that fetched a worker before we cleared the pointers above >>> + * may still be about to queue work on it. Wait for those RCU readers >>> + * to finish before freeing the worker, then run whatever they queued >>> + * so nothing is left with VHOST_WORK_QUEUED set. Mirrors >>> + * vhost_worker_killed(). >>> + */ >>> + if (!xa_empty(&dev->worker_xa)) { >>> + synchronize_rcu(); >>> + xa_for_each(&dev->worker_xa, i, worker) >>> + vhost_run_work_list(worker); >>> + } >>> + >> >> Following sashiko review [1], I tried to undersand why we need this, but >> TBH I'm really confused. That said, this seems wrong also because it >> will work only with vhost_tasks, and not with kthreads. >> >> IIUC vhost_worker_killed() will be called anyway when calling >> vhost_worker_destroy(). For vhost_tasks, it will call >> vhost_task_do_stop() that calls vhost_task_stop(). This sets >> VHOST_TASK_FLAGS_STOP and wait the worker on vtsk->exited before freeing >> stuff. The worker breaks the loop and calls vtsk->handle_sigkill() that >> is exactly vhost_worker_killed() you mentioned we are mirroring here. >> > >Hmm, are we sure it's the case for our codepath? Looking at the >vhost_task loop function: > >> static int vhost_task_fn(void *data) >> { >> for (;;) { >> if (signal_pending(current)) { >> if (get_signal(&ksig)) >> break; >> } >> ... >> if (test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)) { >> __set_current_state(TASK_RUNNING); >> break; >> } >> did_work = vtsk->fn(vtsk->data); >> ... >> } >> >> ... >> >> if (!test_bit(VHOST_TASK_FLAGS_STOP, &vtsk->flags)) { >> set_bit(VHOST_TASK_FLAGS_KILLED, &vtsk->flags); >> vtsk->handle_sigkill(vtsk->data); >> } >> ... >> } > >AFAICT, we exit the loop in 2 cases: signal delivery or STOP bit >setting. Like you said, STOP is set by vhost_task_stop. E.g. for our >RESET_OWNER case: > >vhost_vsock_reset_owner() > vhost_dev_reset_owner() > vhost_dev_cleanup() > vhost_workers_free() > vhost_worker_destroy() > vhost_task_stop() // for vhost_task_ops backend > set_bit(VHOST_TASK_FLAGS_STOP) > >So, first of all, actual work by .fn() callback is done after the exit >checks, therefore we skip it - no chance to drain there. > >Secondly, the handle_sigkill() callback is deliberately NOT called in >the STOP case and only called on fatal signal delivery. And for >vhost_task backend the .handle_sigkill() callback is exactly >vhost_worker_killed(). > >So my understanding is: if we only call synchronize_rcu() here and leave >this path undrained, then whatever work which was put by send_pkt() for >the worker currently being freed - will be lost. Please correct me if >I'm wrong. Yep, your right. But what will be the issue of loosing them? IIUC we are not loosing any data, just avoiding some works that will be handled later when/if will set a new owner. > >That said, I agree that vhost_run_work_list() will only work with >vhost_task backend, not with kthreads backend. If we do >vhost_worker_flush() instead - I guess it'll keep the drain here, yet >become backend-agnostic. I.e.: > >> + if (!xa_empty(&dev->worker_xa)) { >> + synchronize_rcu(); >> + xa_for_each(&dev->worker_xa, i, worker) >> + vhost_worker_flush(worker); >> + } > >With the last 2 lines being equivalent to just calling >vhost_dev_flush(dev). And once we become backend-agnostic here, I'm >guessing the warning reported by Sashiko should be dealt with as well. I'd avoid `if !xa_empty(&dev->worker_xa)` at all, and call synchronize_rcu() in any case. About vhost_dev_flush(), we are calling it in several places, and maybe we should re-check them. E.g. we call in vhost_vsock_flush(), but it's also called by vhost_dev_stop(), maybe we can avoid to call vhost_vsock_flush() if we call vhost_dev_stop(). I'm not sure we really need another one here, but if you think some other works can be queued between the vhost_dev_stop() and the synchronize_rcu() we are adding here, then okay, it may have sense. Thanks, Stefano