Re: [PATCH 1/2] net: adding memory barrier to the poll and receive callbacks

Jarek Poplawski
2009-06-29T17:47:39Z

On Mon, Jun 29, 2009 at 04:14:45PM +0200, Jiri Olsa wrote:
...
> +/**
> + * sk_has_sleeper - check if there are any waiting processes
> + * @sk: socket
> + *
> + * Returns true if socket has waiting processes
> + */
> +static inline int sk_has_sleeper(struct sock *sk)
> +{
> + /*
> + * We need to be sure we are in sync with the
> + * add_wait_queue modifications to the wait queue.
> + *
> + * This memory barrier is paired in the sock_poll_wait.
> + */
> + smp_mb();
> + return sk->sk_sleep && waitqueue_active(sk->sk_sleep);
> +}
> +

Btw. I hope Jiri won't "listen" to me, but I can't stop to mention
sock_waitqueue_active() looks to me quite naturally better paired
with sock_poll_wait() than sk_has_sleeper(which otherwise is more
conspicuous, sorry Eric.)

Jarek P.

> +/**
> + * sock_poll_wait - place memory barrier behind the __poll_wait call.
> + * @filp: file
> + * @sk: socket
> + * @p: poll_table
> + */
> +static inline void sock_poll_wait(struct file *filp, struct sock *sk,
> + poll_table *p)
> +{
> + if (p && sk->sk_sleep) {
> + __poll_wait(filp, sk->sk_sleep, p);
> + /*
> + * We need to be sure we are in sync with the
> + * socket flags modification.
> + *
> + * This memory barrier is paired in the sk_has_sleeper.
> + */
> + smp_mb();
> + }
> +}