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

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Date:   Fri, 22 Sep 2017 18:48:42 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v4 12/13] xen/pvcalls: implement release command


>> +		 */
>> +		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.

Actually, since you don't drop the locks I am not sure recvmsg side will
even get there.

-boris

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