linux-kernel - Re: [PATCH v4 12/13] xen/pvcalls: implement release command

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Date:   Fri, 6 Oct 2017 16:49:55 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v4 12/13] xen/pvcalls: implement release command

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
> 
>