On 7/16/26 11:57 AM, Stefano Garzarella wrote:

On Tue, Jul 14, 2026 at 06:16:37PM +0300, Andrey Drobyshev wrote:

quoted

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?

quoted

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Andrey Drobyshev <redacted>
---
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.

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.

WDYT?

Andrey

So, why we need this?

Should be enough to call synchronize_rcu() in any case after the 
rcu_assign_pointer() loop?

Thanks,
Stefano

[1] 
https://sashiko.dev/#/patchset/20260714151638.143019-1-andrey.drobyshev@virtuozzo.com?part=4