Re: [PATCH v2] ring: check for zero objects mc dequeue / mp enqueue

[Olivier Matz <olivier.matz@6wind.com>]

Hi Lazaros,

On 03/17/2016 04:49 PM, Lazaros Koromilas wrote:
> 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.

Just a question about this patch (that has been applied). Thomas
retitled the commit from your log message:

  ring: fix deadlock in zero object multi enqueue or dequeue
  http://dpdk.org/browse/dpdk/commit/?id=d0979646166e

I think this patch does not fix a deadlock, or did I miss something?

As explained in the following links, the ring may not perform well
if several threads running on the same cpu use it:

  http://dpdk.org/ml/archives/dev/2013-November/000714.html
  http://www.dpdk.org/ml/archives/dev/2014-January/001070.html
  http://www.dpdk.org/ml/archives/dev/2014-January/001162.html
  http://dpdk.org/ml/archives/dev/2015-July/020659.html

A deadlock could occur if the threads running on the same core
have different priority.

Regards,
Olivier