From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751513AbbJGHWa (ORCPT ); Wed, 7 Oct 2015 03:22:30 -0400 Received: from casper.infradead.org ([85.118.1.10]:50185 "EHLO casper.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750947AbbJGHW3 (ORCPT ); Wed, 7 Oct 2015 03:22:29 -0400 Date: Wed, 7 Oct 2015 09:22:27 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, dvhart@linux.intel.com, fweisbec@gmail.com, oleg@redhat.com, bobby.prani@gmail.com Subject: Re: [PATCH tip/core/rcu 10/13] rcu: Add rcu_pointer_handoff() Message-ID: <20151007072227.GV2881@worktop.programming.kicks-ass.net> References: <20151006161305.GA9799@linux.vnet.ibm.com> <1444148028-11551-1-git-send-email-paulmck@linux.vnet.ibm.com> <1444148028-11551-10-git-send-email-paulmck@linux.vnet.ibm.com> <20151006202741.GW3604@twins.programming.kicks-ass.net> <20151006210243.GX3910@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20151006210243.GX3910@linux.vnet.ibm.com> User-Agent: Mutt/1.5.22.1 (2013-10-16) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 06, 2015 at 02:02:43PM -0700, Paul E. McKenney wrote: > On Tue, Oct 06, 2015 at 10:27:41PM +0200, Peter Zijlstra wrote: > > On Tue, Oct 06, 2015 at 09:13:45AM -0700, Paul E. McKenney wrote: > > > /** > > > + * rcu_pointer_handoff() - Hand off a pointer from RCU to other mechanism > > > + * @p: The pointer to hand off > > > + * > > > + * This is simply an identity function, but it documents where a pointer > > > + * is handed off from RCU to some other synchronization mechanism, for > > > + * example, reference counting or locking. In C11, it would map to > > > + * kill_dependency(). It could be used as follows: > > > + * > > > + * rcu_read_lock(); > > > + * p = rcu_dereference(gp); > > > + * long_lived = is_long_lived(p); > > > + * if (long_lived) { > > > + * if (!atomic_inc_not_zero(p->refcnt)) > > > + * long_lived = false; > > > + * else > > > + * p = rcu_pointer_handoff(p); > > > + * } > > > + * rcu_read_unlock(); > > > + */ > > > +#define rcu_pointer_handoff(p) (p) > > > > Will you actually be using this? It seems a tad pointless to add if you > > don't. > > Some of the LLVM guys believe that they can diagnose RCU pointer leaks > if this is used. But yes, it does need to be used. The thing is, I'm not convinced this is a 'sane' interface. Its _far_ too easy to forget. It doesn't make any kind of sense either, which is part of why its hard to remember.