From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751641AbaELAGP (ORCPT ); Sun, 11 May 2014 20:06:15 -0400 Received: from mail-pa0-f53.google.com ([209.85.220.53]:43830 "EHLO mail-pa0-f53.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751472AbaELAGO (ORCPT ); Sun, 11 May 2014 20:06:14 -0400 Date: Sun, 11 May 2014 17:06:19 -0700 From: Kent Overstreet To: Tejun Heo Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH percpu/for-3.16 2/2] percpu-refcount: implement percpu_ref_tryget() Message-ID: <20140512000619.GC10617@kmo-pixel> References: <20140507155511.GB26540@htj.dyndns.org> <20140507155647.GC26540@htj.dyndns.org> <20140507155810.GD26540@htj.dyndns.org> <20140509194020.GB2276@kmo> <20140509194140.GC4486@htj.dyndns.org> <20140509195109.GC2276@kmo> <20140509195522.GF4486@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140509195522.GF4486@htj.dyndns.org> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, May 09, 2014 at 03:55:22PM -0400, Tejun Heo wrote: > Hello, > > On Fri, May 09, 2014 at 12:51:09PM -0700, Kent Overstreet wrote: > > Well not so much deprecated as "bad, avoid" - IMO using tryget() almost always > > (I haven't seen a convincing counterexample) means you screwed up your > > refcounting somewhere, if you need to take a ref on something whatever made that > > object visible to you should have its own ref. > > > > (I think we had this debate, but that was awhile ago...) > > Oh sure, tryget can definitely be misunderstood but RCU protected > iteration is one valid use case. > > rcu_read_lock(); > locate the object of interest; > tryget[_live]() depending on the use case; > rcu_read_unlock(); > > access the object. No, it's not needed with RCU... look at the aio code for an example (or don't, save your eyes instead). Conceptually the RCU data structure should own a refcount on the things that are accessible via it; that ref shouldn't be dropped until after it's removed and an RCU barrier has happened.