From mboxrd@z Thu Jan 1 00:00:00 1970 From: Neil Horman Subject: Re: [PATCH] mbuf: add comment explaining confusing code Date: Fri, 27 Mar 2015 06:29:56 -0400 Message-ID: <20150327102956.GB5375@hmsreliant.think-freely.org> References: <1427404494-27256-1-git-send-email-bruce.richardson@intel.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: dev-VfR2kkLFssw@public.gmane.org To: Bruce Richardson Return-path: Content-Disposition: inline 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" 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 > --- > 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