[dpdk-dev] [PATCH v2] ring: check for zero objects mc dequeue / mp enqueue

Lazaros Koromilas l at nofutznetworks.com
Thu Mar 17 16:49:43 CET 2016


Issuing a zero objects dequeue with a single consumer has no effect.
Doing so with multiple consumers, can get more than one thread to succeed
the compare-and-set operation and observe starvation or even deadlock in
the while loop that checks for preceding dequeues.  The problematic piece
of code when n = 0:

    cons_next = cons_head + n;
    success = rte_atomic32_cmpset(&r->cons.head, cons_head, cons_next);

The same is possible on the enqueue path.

Signed-off-by: Lazaros Koromilas <l at nofutznetworks.com>
---
 lib/librte_ring/rte_ring.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index 943c97c..eb45e41 100644
--- a/lib/librte_ring/rte_ring.h
+++ b/lib/librte_ring/rte_ring.h
@@ -431,6 +431,11 @@ __rte_ring_mp_do_enqueue(struct rte_ring *r, void * const *obj_table,
 	uint32_t mask = r->prod.mask;
 	int ret;
 
+	/* Avoid the unnecessary cmpset operation below, which is also
+	 * potentially harmful when n equals 0. */
+	if (n == 0)
+		return 0;
+
 	/* move prod.head atomically */
 	do {
 		/* Reset n to the initial burst count */
@@ -618,6 +623,11 @@ __rte_ring_mc_do_dequeue(struct rte_ring *r, void **obj_table,
 	unsigned i, rep = 0;
 	uint32_t mask = r->prod.mask;
 
+	/* Avoid the unnecessary cmpset operation below, which is also
+	 * potentially harmful when n equals 0. */
+	if (n == 0)
+		return 0;
+
 	/* move cons.head atomically */
 	do {
 		/* Restore n as it may change every loop */
-- 
1.9.1



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