linux-kernel - Re: [PATCH v6 12/13] xen/pvcalls: implement release command

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Date:   Wed, 25 Oct 2017 16:00:40 -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 v6 12/13] xen/pvcalls: implement release command

On Wed, 25 Oct 2017, Boris Ostrovsky wrote:
> On 10/24/2017 01:33 PM, Stefano Stabellini wrote:
> > Send PVCALLS_RELEASE to the backend and wait for a reply. Take both
> > in_mutex and out_mutex to avoid concurrent accesses. Then, free the
> > socket.
> >
> > For passive sockets, check whether we have already pre-allocated an
> > active socket for the purpose of being accepted. If so, free that as
> > well.
> >
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-front.c | 100 ++++++++++++++++++++++++++++++++++++++++++++
> >  drivers/xen/pvcalls-front.h |   1 +
> >  2 files changed, 101 insertions(+)
> >
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 4a413ff..7abc039 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -199,6 +199,23 @@ static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
> >  static void pvcalls_front_free_map(struct pvcalls_bedata *bedata,
> >  				   struct sock_mapping *map, bool locked)
> >  {
> > +	int i;
> > +
> > +	unbind_from_irqhandler(map->active.irq, map);
> > +
> > +	if (!locked)
> > +		spin_lock(&bedata->socket_lock);
> > +	if (!list_empty(&map->list))
> > +		list_del_init(&map->list);
> > +	if (!locked)
> > +		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);
> > +
> > +	kfree(map);
> >  }
> >  
> >  static irqreturn_t pvcalls_front_conn_handler(int irq, void *sock_map)
> > @@ -966,6 +983,89 @@ 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;
> > +
> 
> ..
> 
> > +
> > +	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();
> > +
> > +		pvcalls_front_free_map(bedata, map, false);
> > +	} else {
> > +		spin_lock(&bedata->socket_lock);
> > +		if (READ_ONCE(map->passive.inflight_req_id) !=
> > +		    PVCALLS_INVALID_ID) {
> > +			pvcalls_front_free_map(bedata,
> > +					       map->passive.accept_map, true);
> > +		}
> > +		list_del(&map->list);
> > +		kfree(map);
> > +		spin_unlock(&bedata->socket_lock);
> 
> We have different locking rules in pvcalls_front_free_map() for each of
> those clauses in that in the first case we are doing grant table
> operations and free_page() without the lock and in the second case we
> are holding it. Is it possible to restructure this so that we prune the
> lists under the lock (possibly in this routine) and call
> pvcalls_front_free_map() lock-less?

Yes, it is possible. However, pvcalls_front_free_map is called from a
couple of other places (pvcalls_front_accept and pvcalls_front_remove)
and we would have to add the code to remove the map from the list there
as well. I am not sure it is worth it.

I don't have a strong opinion on this. Let me know which way you prefer.