vhost: synchronize with RCU readers when freeing workers

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 threading a bool 'sync' param through the chain:

  vhost_dev_reset_owner()
    vhost_dev_cleanup()
      vhost_workers_free()

When set, after clearing the vq->worker pointers, wait for a grace
period and flush the workers so any work the last RCU readers queued
runs (clearing VHOST_WORK_QUEUED) before the workers are freed.  Only
the vsock RESET_OWNER path (added in the next patch) passes sync=true;
every other teardown has already quiesced and passes false, so they do
not need to wait the grace period.

Suggested-by: Stefano Garzarella <sgarzare@redhat.com>
Suggested-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260724114542.734623-5-andrey.drobyshev@virtuozzo.com>

0 files changed, 0 insertions, 0 deletions