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. Suggested-by: Stefano Garzarella <[email protected]> Signed-off-by: Andrey Drobyshev <[email protected]> --- 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); + } + /* * Free the default worker we created and cleanup workers userspace * created but couldn't clean up (it forgot or crashed). -- 2.47.1
