Re: [PATCH v5 4/5] vhost: synchronize with RCU readers when freeing workers

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Jul 20, 2026 at 01:22:40PM +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.
>
>Fix this by clearing the vq->worker pointers, waiting for a grace
>period, and then flushing the workers so any work the last readers
>queued runs before the workers are freed.
>
>Fixes: 228a27cf78af ("vhost: Allow worker switching while work is queueing")
>Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
>Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
>---
> drivers/vhost/vhost.c | 11 +++++++++++
> 1 file changed, 11 insertions(+)

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>