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

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Message-ID: <alpine.DEB.1.10.0906290826300.29966@makko.or.mcafeemobile.com>
Date:	Mon, 29 Jun 2009 08:34:55 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Jiri Olsa <jolsa@...hat.com>
cc:	netdev@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	fbl@...hat.com, nhorman@...hat.com, davem@...hat.com,
	htejun@...il.com, jarkao2@...il.com, oleg@...hat.com,
	eric.dumazet@...il.com
Subject: Re: [PATCH 1/2] net: adding memory barrier to the poll and receive
 callbacks

On Mon, 29 Jun 2009, Jiri Olsa wrote:

> -static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
> +static inline void __poll_wait(struct file *filp,
> +				wait_queue_head_t *wait_address, poll_table *p)
> +{
> +	p->qproc(filp, wait_address, p);
> +}
> +
> +static inline void poll_wait(struct file *filp,
> +				wait_queue_head_t *wait_address, poll_table *p)
>  {
>  	if (p && wait_address)
> -		p->qproc(filp, wait_address, p);
> +		__poll_wait(filp, wait_address, p);
>  }

> +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();
> +	}
> +}

I think Oleg already said this, but you can use directly poll_wait() 
without adding another abstraction, and the compiler will drop the double 
check for you:

extern void foo(int, int, int);
extern void mb(void);

static inline void cfoo(int a, int b, int c) {
	if (b && c)
		foo(a, b, c);
}

void xxx(int a, int b, int c) {
	if (b && c) {
		cfoo(a, b, c);
		mb();
	}
}
-----
xxx:
	subq	$8, %rsp
	testl	%esi, %esi
	je	.L3
	testl	%edx, %edx
	je	.L3
	call	foo
	addq	$8, %rsp
	jmp	mb
.L3:
	addq	$8, %rsp
	ret



- Davide


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