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SubjectRe: [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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