[dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue

[Jia He]

Before this patch:
In __rte_ring_move_cons_head()
...
        do {
                /* Restore n as it may change every loop */
                n = max;

                *old_head = r->cons.head;                //1st load
                const uint32_t prod_tail = r->prod.tail; //2nd load

In weak memory order architectures(powerpc,arm), the 2nd load might be
reodered before the 1st load, that makes *entries is bigger than we wanted.
This nasty reording messed enque/deque up.

cpu1(producer)          cpu2(consumer)          cpu3(consumer)
                        load r->prod.tail
in enqueue:
load r->cons.tail
load r->prod.head

store r->prod.tail

                                                load r->cons.head
                                                load r->prod.tail
                                                ...
                                                store r->cons.{head,tail}
                        load r->cons.head

THEN,r->cons.head will be bigger than prod_tail, then make *entries very big

After this patch, the old cons.head will be recaculated after failure of
rte_atomic32_cmpset

There is no such issue in X86 cpu, because X86 is strong memory order model

Signed-off-by: Jia He <hejianet at gmail.com>
Signed-off-by: jia.he at hxt-semitech.com
Signed-off-by: jie2.liu at hxt-semitech.com
Signed-off-by: bing.zhao at hxt-semitech.com

---
 lib/librte_ring/rte_ring.h | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index 5e9b3b7..15c72e2 100644
--- a/lib/librte_ring/rte_ring.h
+++ b/lib/librte_ring/rte_ring.h
@@ -409,6 +409,10 @@ __rte_ring_move_prod_head(struct rte_ring *r, int is_sp,
 		n = max;
 
 		*old_head = r->prod.head;
+
+		/* load of prod.tail can't be reordered before cons.head */
+		rte_smp_rmb();
+
 		const uint32_t cons_tail = r->cons.tail;
 		/*
 		 *  The subtraction is done between two unsigned 32bits value
@@ -517,6 +521,10 @@ __rte_ring_move_cons_head(struct rte_ring *r, int is_sc,
 		n = max;
 
 		*old_head = r->cons.head;
+
+		/* load of prod.tail can't be reordered before cons.head */
+		rte_smp_rmb();
+
 		const uint32_t prod_tail = r->prod.tail;
 		/* The subtraction is done between two unsigned 32bits value
 		 * (the result is always modulo 32 bits even if we have
-- 
2.7.4