Re: [Xen-devel] [PATCH v4 12/13] xen/pvcalls: implement release command

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 22 Sep 2017, Boris Ostrovsky wrote:
> > +static void pvcalls_front_free_map(struct pvcalls_bedata *bedata,
> > +                              struct sock_mapping *map)
> 
> I just noticed: pvcalls_front_free_map() is referenced by patches 2 and 8.

I'll create an empty stub earlier.


> > +{
> > +   int i;
> > +
> > +   unbind_from_irqhandler(map->active.irq, map);
> > +
> > +   spin_lock(&bedata->socket_lock);
> > +   if (!list_empty(&map->list))
> > +           list_del_init(&map->list);
> > +   spin_unlock(&bedata->socket_lock);
> > +
> > +   for (i = 0; i < (1 << PVCALLS_RING_ORDER); i++)
> > +           gnttab_end_foreign_access(map->active.ring->ref[i], 0, 0);
> > +   gnttab_end_foreign_access(map->active.ref, 0, 0);
> > +   free_page((unsigned long)map->active.ring);
> > +}
> > +
> >  int pvcalls_front_socket(struct socket *sock)
> >  {
> >     struct pvcalls_bedata *bedata;
> > @@ -960,6 +978,92 @@ unsigned int pvcalls_front_poll(struct file *file, 
> > struct socket *sock,
> >     return ret;
> >  }
> >  
> > +int pvcalls_front_release(struct socket *sock)
> > +{
> > +   struct pvcalls_bedata *bedata;
> > +   struct sock_mapping *map;
> > +   int req_id, notify, ret;
> > +   struct xen_pvcalls_request *req;
> > +
> > +   pvcalls_enter;
> > +   if (!pvcalls_front_dev) {
> > +           pvcalls_exit;
> > +           return -EIO;
> > +   }
> > +   if (sock->sk == NULL) {
> > +           pvcalls_exit;
> > +           return 0;
> > +   }
> > +
> > +   bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +
> > +   map = (struct sock_mapping *) sock->sk->sk_send_head;
> > +   if (map == NULL) {
> > +           pvcalls_exit;
> > +           return 0;
> > +   }
> > +
> > +   spin_lock(&bedata->socket_lock);
> > +   ret = get_request(bedata, &req_id);
> > +   if (ret < 0) {
> > +           spin_unlock(&bedata->socket_lock);
> > +           pvcalls_exit;
> > +           return ret;
> > +   }
> > +   sock->sk->sk_send_head = NULL;
> > +
> > +   req = RING_GET_REQUEST(&bedata->ring, req_id);
> > +   req->req_id = req_id;
> > +   req->cmd = PVCALLS_RELEASE;
> > +   req->u.release.id = (uint64_t)map;
> > +
> > +   bedata->ring.req_prod_pvt++;
> > +   RING_PUSH_REQUESTS_AND_CHECK_NOTIFY(&bedata->ring, notify);
> > +   spin_unlock(&bedata->socket_lock);
> > +   if (notify)
> > +           notify_remote_via_irq(bedata->irq);
> > +
> > +   wait_event(bedata->inflight_req,
> > +              READ_ONCE(bedata->rsp[req_id].req_id) == req_id);
> > +
> > +   if (map->active_socket) {
> > +           /*
> > +            * Set in_error and wake up inflight_conn_req to force
> > +            * recvmsg waiters to exit.
> > +            */
> > +           map->active.ring->in_error = -EBADF;
> > +           wake_up_interruptible(&map->active.inflight_conn_req);
> > +
> > +           /*
> > +            * Wait until there are no more waiters on the mutexes.
> > +            * We know that no new waiters can be added because sk_send_head
> > +            * is set to NULL -- we only need to wait for the existing
> > +            * waiters to return.
> > +            */
> > +           while (!mutex_trylock(&map->active.in_mutex) ||
> > +                      !mutex_trylock(&map->active.out_mutex))
> > +                   cpu_relax();
> 
> 
> What if you manage to grab the locks before waiters get to run? for
> example, in recvmsg:
> 
>       while (!(flags & MSG_DONTWAIT) && !pvcalls_front_read_todo(map)) {
>               wait_event_interruptible(map->active.inflight_conn_req,
>                                        pvcalls_front_read_todo(map));
>       }
>       ret = __read_ring(map->active.ring, &map->active.data,
>                         &msg->msg_iter, len, flags);
> 
> map will be freed (by pvcalls_front_free_map() below) before __read_ring
> is passed the just-freed ring.

I don't think that is possible: mutex_trylock should return 0 on
contention. I don't think is possible for mutex_trylock to return 1 when
there are waiters waiting for their chance to run.



 
> > +
> > +           pvcalls_front_free_map(bedata, map);
> > +           kfree(map);
> 
> 
> -boris
> 
> 

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