Date: Thu, 7 May 2015 18:10:36 +0800 From: rhenjau <rhenjau@gmail.com> To: freebsd-hackers@freebsd.org Subject: Memory barriers about buf_ring(9) Message-ID: <CA%2BO7MXzRuy33tWQRWAnh3Z6ZsdLOsVvm0-1p8%2BjrfFOannjZbQ@mail.gmail.com>
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Hi, hackers
I'm reading buf_ring(9) and have a question about the following
function. Is a memory barrier needed before br_cons_tail to prevent the
loading of br_ring[cons_head] load is reordered after br_cons_tail is set?
Producers may overwrite the area.
/*
* single-consumer dequeue
* use where dequeue is protected by a lock
* e.g. a network driver's tx queue lock
*/
static __inline void *
buf_ring_dequeue_sc(struct buf_ring *br)
{
uint32_t cons_head, cons_next;
#ifdef PREFETCH_DEFINED
uint32_t cons_next_next;
#endif
uint32_t prod_tail;
void *buf;
cons_head = br->br_cons_head;
prod_tail = br->br_prod_tail;
cons_next = (cons_head + 1) & br->br_cons_mask;
#ifdef PREFETCH_DEFINED
cons_next_next = (cons_head + 2) & br->br_cons_mask;
#endif
if (cons_head == prod_tail)
return (NULL);
#ifdef PREFETCH_DEFINED
if (cons_next != prod_tail) {
prefetch(br->br_ring[cons_next]);
if (cons_next_next != prod_tail)
prefetch(br->br_ring[cons_next_next]);
}
#endif
br->br_cons_head = cons_next;
buf = br->br_ring[cons_head];
#ifdef DEBUG_BUFRING
br->br_ring[cons_head] = NULL;
if (!mtx_owned(br->br_lock))
panic("lock not held on single consumer dequeue");
if (br->br_cons_tail != cons_head)
panic("inconsistent list cons_tail=%d cons_head=%d",
br->br_cons_tail, cons_head);
#endif
----------------------------------------------------- need memory
barrier?
br->br_cons_tail = cons_next;
return (buf);
}
--
-rhenjau
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