Re: [dpdk-dev] [PATCH] mbuf: add comment explaining confusing code

[Neil Horman <nhorman@tuxdriver.com>]

On Thu, Mar 26, 2015 at 09:14:54PM +0000, Bruce Richardson wrote:
> The logic used in the condition check before freeing an mbuf is
> sometimes confusing, so explain it in a proper comment.
> 
> Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> ---
>  lib/librte_mbuf/rte_mbuf.h | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 17ba791..0265172 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -764,6 +764,16 @@ __rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>  {
>  	__rte_mbuf_sanity_check(m, 0);
>  
> +	/*
> +	 * Check to see if this is the last reference to the mbuf.
> +	 * Note: the double check here is deliberate. If the ref_cnt is "atomic"
> +	 * the call to "refcnt_update" is a very expensive operation, so we
> +	 * don't want to call it in the case where we know we are the holder
> +	 * of the last reference to this mbuf i.e. ref_cnt == 1.
> +	 * If however, ref_cnt != 1, it's still possible that we may still be
> +	 * the final decrementer of the count, so we need to check that
> +	 * result also, to make sure the mbuf is freed properly.
> +	 */
>  	if (likely (rte_mbuf_refcnt_read(m) == 1) ||
>  			likely (rte_mbuf_refcnt_update(m, -1) == 0)) {
>  
> -- 
> 2.1.0
> 
> 

NAK
 the comment is incorrect, a return code of 1 from rte_mbuf_refcnt_read doesn't
guarantee you are the last holder of the buffer if two contexts have a pointer
to it.

Zoltans patch is the correct solution here, expensive or not.  I wrote up my
explination in this thread:
http://dpdk.org/ml/archives/dev/2015-March/015839.html