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diff --git a/sys/sys/buf_ring.h b/sys/sys/buf_ring.h
--- a/sys/sys/buf_ring.h
+++ b/sys/sys/buf_ring.h
@@ -131,7 +131,7 @@
buf_ring_dequeue_mc(struct buf_ring *br)
{
uint32_t cons_head, cons_next, cons_idx;
- uint32_t mask;
+ uint32_t prod_tail, mask;
void *buf;
critical_enter();
@@ -139,8 +139,9 @@
do {
cons_head = br->br_cons_head;
cons_next = cons_head + 1;
+ prod_tail = atomic_load_acq_32(&br->br_prod_tail);
- if (cons_head == br->br_prod_tail) {
+ if (cons_head == prod_tail) {
critical_exit();
return (NULL);
}
@@ -266,29 +267,26 @@
static __inline void *
buf_ring_peek(struct buf_ring *br)
{
- uint32_t mask;
+ uint32_t cons_head, prod_tail, mask;
#if defined(DEBUG_BUFRING) && defined(_KERNEL)
if ((br->br_lock != NULL) && !mtx_owned(br->br_lock))
panic("lock not held on single consumer dequeue");
#endif
mask = br->br_cons_mask;
- /*
- * I believe it is safe to not have a memory barrier
- * here because we control cons and tail is worst case
- * a lagging indicator so we worst case we might
- * return NULL immediately after a buffer has been enqueued
- */
- if (br->br_cons_head == br->br_prod_tail)
+ prod_tail = atomic_load_acq_32(&br->br_prod_tail);
+ cons_head = br->br_cons_head;
+
+ if (cons_head == prod_tail)
return (NULL);
- return (br->br_ring[br->br_cons_head & mask]);
+ return (br->br_ring[cons_head & mask]);
}
static __inline void *
buf_ring_peek_clear_sc(struct buf_ring *br)
{
- uint32_t mask;
+ uint32_t cons_head, prod_tail, mask;
void *ret;
#if defined(DEBUG_BUFRING) && defined(_KERNEL)
@@ -297,30 +295,19 @@
#endif
mask = br->br_cons_mask;
- if (br->br_cons_head == br->br_prod_tail)
- return (NULL);
+ prod_tail = atomic_load_acq_32(&br->br_prod_tail);
+ cons_head = br->br_cons_head;
-#if defined(__arm__) || defined(__aarch64__)
- /*
- * The barrier is required there on ARM and ARM64 to ensure, that
- * br->br_ring[br->br_cons_head] will not be fetched before the above
- * condition is checked.
- * Without the barrier, it is possible, that buffer will be fetched
- * before the enqueue will put mbuf into br, then, in the meantime, the
- * enqueue will update the array and the br_prod_tail, and the
- * conditional check will be true, so we will return previously fetched
- * (and invalid) buffer.
- */
- atomic_thread_fence_acq();
-#endif
+ if (cons_head == prod_tail)
+ return (NULL);
- ret = br->br_ring[br->br_cons_head & mask];
+ ret = br->br_ring[cons_head & mask];
#ifdef DEBUG_BUFRING
/*
* Single consumer, i.e. cons_head will not move while we are
* running, so atomic_swap_ptr() is not necessary here.
*/
- br->br_ring[br->br_cons_head & mask] = NULL;
+ br->br_ring[cons_head & mask] = NULL;
#endif
return (ret);
}

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