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* [dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue
@ 2017-10-10  9:56 Jia He
  2017-10-12 15:53 ` Olivier MATZ
  0 siblings, 1 reply; 41+ messages in thread
From: Jia He @ 2017-10-10  9:56 UTC (permalink / raw)
  To: olivier.matz; +Cc: dev, Jia He, jia.he, jie2.liu, bing.zhao

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@gmail.com>
Signed-off-by: jia.he@hxt-semitech.com
Signed-off-by: jie2.liu@hxt-semitech.com
Signed-off-by: bing.zhao@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

^ permalink raw reply	[flat|nested] 41+ messages in thread

end of thread, other threads:[~2017-11-03 12:48 UTC | newest]

Thread overview: 41+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2017-10-10  9:56 [dpdk-dev] [PATCH] ring: guarantee ordering of cons/prod loading when doing enqueue/dequeue Jia He
2017-10-12 15:53 ` Olivier MATZ
2017-10-12 16:15   ` Stephen Hemminger
2017-10-12 17:05   ` Ananyev, Konstantin
2017-10-12 17:23     ` Jerin Jacob
2017-10-13  1:02       ` Jia He
2017-10-13  1:15         ` Jia He
2017-10-13  1:16         ` Jia He
2017-10-13  1:49           ` Jerin Jacob
2017-10-13  3:23             ` Jia He
2017-10-13  5:57               ` Zhao, Bing
2017-10-13  7:33             ` Jianbo Liu
2017-10-13  8:20               ` Jia He
2017-10-19 10:02           ` Ananyev, Konstantin
2017-10-19 11:18             ` Zhao, Bing
2017-10-19 14:15               ` Ananyev, Konstantin
2017-10-19 20:02                 ` Ananyev, Konstantin
2017-10-20  1:57                   ` Jia He
2017-10-20  5:43                     ` Jerin Jacob
2017-10-23  8:49                       ` Jia He
2017-10-23  9:05                         ` Kuusisaari, Juhamatti
2017-10-23  9:10                           ` Bruce Richardson
2017-10-23 10:06                         ` Jerin Jacob
2017-10-24  2:04                           ` Jia He
2017-10-25 13:26                             ` Jerin Jacob
2017-10-26  2:27                               ` Jia He
2017-10-31  2:55                               ` Jia He
2017-10-31 11:14                                 ` Jerin Jacob
2017-11-01  2:53                                   ` Jia He
2017-11-01 19:04                                     ` Jerin Jacob
2017-11-02  1:09                                       ` Jia He
2017-11-02  8:57                                       ` Jia He
2017-11-03  2:55                                         ` Jia He
2017-11-03 12:47                                           ` Jerin Jacob
2017-11-01  4:48                                   ` Jia He
2017-11-01 19:10                                     ` Jerin Jacob
2017-10-20  7:03                     ` Ananyev, Konstantin
2017-10-13  0:24     ` Liu, Jie2
2017-10-13  2:12       ` Zhao, Bing
2017-10-13  2:34         ` Jerin Jacob
2017-10-16 10:51       ` Kuusisaari, Juhamatti

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