ring: fix deadlock in zero object multi enqueue or dequeue

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.

Fixes: af75078fece3 ("first public release")

Signed-off-by: Lazaros Koromilas <l@nofutznetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>

diff --git a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h
index 943c97c..eb45e41 100644 (file)
--- 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 */