Re: [Xen-devel] [PATCH] xen/pvcalls: fix potential endless loop in pvcalls-front.c

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/15/2017 02:09 PM, Stefano Stabellini wrote:
> On Wed, 15 Nov 2017, Juergen Gross wrote:
>>>>>         while(mutex_is_locked(&map->active.in_mutex.owner) ||
>>>>>               mutex_is_locked(&map->active.out_mutex.owner))
>>>>>                 cpu_relax();
>>>>>
>>>>> ?
>>>> I'm not convinced there isn't a race.
>>>>
>>>> In pvcalls_front_recvmsg() sock->sk->sk_send_head is being read and only
>>>> then in_mutex is taken. What happens if pvcalls_front_release() resets
>>>> sk_send_head and manages to test the mutex before the mutex is locked?
>>>>
>>>> Even in case this is impossible: the whole construct seems to be rather
>>>> fragile.
> I agree it looks fragile, and I agree that it might be best to avoid the
> usage of in_mutex and out_mutex as refcounts. More comments on this
> below.
>
>  
>>> I think we can wait until pvcalls_refcount is 1 (i.e. it's only us) and
>>> not rely on mutex state.
>> Yes, this would work.
> Yes, I agree it would work and for the sake of getting something in
> shape for the merge window I am attaching a patch for it. Please go
> ahead with it. Let me know if you need anything else immediately, and
> I'll work on it ASAP.
>
>
>
> However, I should note that this is a pretty big hammer we are using:
> the refcount is global, while we only need to wait until it's only us
> _on this specific socket_.

Can you explain why socket is important?

>
> We really need a per socket refcount. If we don't want to use the mutex
> internal counters, then we need another one.
>
> See the appended patch that introduces a per socket refcount. However,
> for the merge window, also using pvcalls_refcount is fine.
>
> The race Juergen is concerned about is only theoretically possible:
>
> recvmsg:                                 release:
>   
>   test sk_send_head                      clear sk_send_head
>   <only few instructions>                <prepare a message>
>   grab in_mutex                          <send a message to the server>
>                                          <wait for an answer>
>                                          test in_mutex
>
> Without kernel preemption is not possible for release to clear
> sk_send_head and test in_mutex after recvmsg tests sk_send_head and
> before recvmsg grabs in_mutex.

Sorry, I don't follow --- what does preemption have to do with this? If
recvmsg and release happen on different processors the order of
operations can be

CPU0                                   CPU1

test sk_send_head
<interrupt>
                                        clear sk_send_head
                                        <send a message to the server>
                                        <wait for an answer>
                                        test in_mutex
                                        free everything
grab in_mutex

I actually think RCU should take care of all of this.

But for now I will take your refcount-based patch. However, it also
needs comment update.

How about

/*
 * We need to make sure that send/rcvmsg on this socket has not started
 * before we've cleared sk_send_head here. The easiest (though not optimal)
 * way to guarantee this is to see that no pvcall (other than us) is in
progress.
 */

-boris


>
> But maybe we need to disable kernel preemption in recvmsg and sendmsg to
> stay on the safe side?
>
> The patch below introduces a per active socket refcount, so that we
> don't have to rely on in_mutex and out_mutex for refcounting. It also
> disables preemption in sendmsg and recvmsg in the region described
> above.
>
> I don't think this patch should go in immediately. We can take our time
> to figure out the best fix.
>
>
> diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> index 0c1ec68..8c1030b 100644
> --- a/drivers/xen/pvcalls-front.c
> +++ b/drivers/xen/pvcalls-front.c
> @@ -68,6 +68,7 @@ struct sock_mapping {
>                       struct pvcalls_data data;
>                       struct mutex in_mutex;
>                       struct mutex out_mutex;
> +                     atomic_t sock_refcount;
>  
>                       wait_queue_head_t inflight_conn_req;
>               } active;
> @@ -497,15 +498,20 @@ int pvcalls_front_sendmsg(struct socket *sock, struct 
> msghdr *msg,
>       }
>       bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
>  
> +     preempt_disable();
>       map = (struct sock_mapping *) sock->sk->sk_send_head;
>       if (!map) {
> +             preempt_enable();
>               pvcalls_exit();
>               return -ENOTSOCK;
>       }
>  
> +     atomic_inc(&map->active.sock_refcount);
>       mutex_lock(&map->active.out_mutex);
> +     preempt_enable();
>       if ((flags & MSG_DONTWAIT) && !pvcalls_front_write_todo(map)) {
>               mutex_unlock(&map->active.out_mutex);
> +             atomic_dec(&map->active.sock_refcount);
>               pvcalls_exit();
>               return -EAGAIN;
>       }
> @@ -528,6 +534,7 @@ int pvcalls_front_sendmsg(struct socket *sock, struct 
> msghdr *msg,
>               tot_sent = sent;
>  
>       mutex_unlock(&map->active.out_mutex);
> +     atomic_dec(&map->active.sock_refcount);
>       pvcalls_exit();
>       return tot_sent;
>  }
> @@ -600,13 +607,17 @@ int pvcalls_front_recvmsg(struct socket *sock, struct 
> msghdr *msg, size_t len,
>       }
>       bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
>  
> +     preempt_disable();
>       map = (struct sock_mapping *) sock->sk->sk_send_head;
>       if (!map) {
> +             preempt_enable();
>               pvcalls_exit();
>               return -ENOTSOCK;
>       }
>  
> +     atomic_inc(&map->active.sock_refcount);
>       mutex_lock(&map->active.in_mutex);
> +     preempt_enable();
>       if (len > XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER))
>               len = XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
>  
> @@ -625,6 +636,7 @@ int pvcalls_front_recvmsg(struct socket *sock, struct 
> msghdr *msg, size_t len,
>               ret = 0;
>  
>       mutex_unlock(&map->active.in_mutex);
> +     atomic_dec(&map->active.sock_refcount);
>       pvcalls_exit();
>       return ret;
>  }
> @@ -1048,8 +1060,7 @@ int pvcalls_front_release(struct socket *sock)
>                * 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))
> +             while (atomic_read(&map->active.sock_refcount) > 0)
>                       cpu_relax();
>  
>               pvcalls_front_free_map(bedata, map);


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