From mboxrd@z Thu Jan 1 00:00:00 1970 From: Olivier MATZ Subject: Re: [PATCH] mbuf: add comment explaining confusing code Date: Thu, 26 Mar 2015 22:31:01 +0100 Message-ID: <55147A95.6060102@6wind.com> References: <1427404494-27256-1-git-send-email-bruce.richardson@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit To: Bruce Richardson , dev-VfR2kkLFssw@public.gmane.org Return-path: In-Reply-To: <1427404494-27256-1-git-send-email-bruce.richardson-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces-VfR2kkLFssw@public.gmane.org Sender: "dev" Hi Bruce, On 03/26/2015 10:14 PM, 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 > --- > 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)) { > > Acked-by: Olivier Matz Thanks!