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ring: fix deadlock in zero object multi enqueue or dequeue

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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: af75078fec ("first public release")

Signed-off-by: Lazaros Koromilas <l@nofutznetworks.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>
This commit is contained in:
Lazaros Koromilas 2016-03-17 17:49:43 +02:00 committed by Thomas Monjalon
parent 768f0e4587
commit d097964616
1 changed files with 10 additions and 0 deletions
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10
lib/librte_ring/rte_ring.h
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@ -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 */