From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp.tuxdriver.com (charlotte.tuxdriver.com [70.61.120.58]) by dpdk.org (Postfix) with ESMTP id 0AB7E5A26 for ; Fri, 27 Mar 2015 13:42:16 +0100 (CET) Received: from hmsreliant.think-freely.org ([2001:470:8:a08:7aac:c0ff:fec2:933b] helo=localhost) by smtp.tuxdriver.com with esmtpsa (TLSv1:AES128-SHA:128) (Exim 4.63) (envelope-from ) id 1YbTaW-0005Z9-FO; Fri, 27 Mar 2015 08:42:14 -0400 Date: Fri, 27 Mar 2015 08:42:11 -0400 From: Neil Horman To: Bruce Richardson Message-ID: <20150327124211.GD5375@hmsreliant.think-freely.org> References: <1427404494-27256-1-git-send-email-bruce.richardson@intel.com> <20150327102956.GB5375@hmsreliant.think-freely.org> <20150327113238.GA11660@bricha3-MOBL3> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150327113238.GA11660@bricha3-MOBL3> User-Agent: Mutt/1.5.23 (2014-03-12) X-Spam-Score: -2.9 (--) X-Spam-Status: No Cc: dev@dpdk.org Subject: Re: [dpdk-dev] [PATCH] mbuf: add comment explaining confusing code X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: patches and discussions about DPDK List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 27 Mar 2015 12:42:16 -0000 On Fri, Mar 27, 2015 at 11:32:38AM +0000, Bruce Richardson wrote: > On Fri, Mar 27, 2015 at 06:29:56AM -0400, Neil Horman wrote: > > 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. > If two threads have pointers to it, and are both going to free it, the refcnt > must be 2 not one, otherwise the refcnt is meaningless. > Please see my note in zoltans thread. I illustrate why its important to fix this in several ways. Relying of every context to have a refcnt on an object before having pointer reference to it is impossible to do without additional locking. > /Bruce > > > > > 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 > > > > >