Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751024AbaJ1E1C (ORCPT ); Tue, 28 Oct 2014 00:27:02 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:51478 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756170AbaJ1E06 (ORCPT ); Tue, 28 Oct 2014 00:26:58 -0400 Date: Mon, 27 Oct 2014 21:11:27 -0700 From: "Paul E. McKenney" To: Al Viro Cc: Miklos Szeredi , Linus Torvalds , Linux-Fsdevel , Kernel Mailing List , linux-unionfs@vger.kernel.org Subject: Re: [GIT PULL] overlay filesystem v25 Message-ID: <20141028041127.GM5718@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20141024032422.GI7996@ZenIV.linux.org.uk> <20141025081845.GJ7996@ZenIV.linux.org.uk> <20141025170609.GK7996@ZenIV.linux.org.uk> <20141027155901.GE5718@linux.vnet.ibm.com> <20141027172800.GW7996@ZenIV.linux.org.uk> <20141027173621.GG5718@linux.vnet.ibm.com> <20141028011214.GZ7996@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141028011214.GZ7996@ZenIV.linux.org.uk> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14102804-0033-0000-0000-0000027C56B5 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 28, 2014 at 01:12:14AM +0000, Al Viro wrote: > On Mon, Oct 27, 2014 at 10:36:21AM -0700, Paul E. McKenney wrote: > > Code making direct use of smp_read_barrier_depends() is harder to read, > > in my experience, but good point on the sparse noise. Maybe a new > > lockless_dereference() primitive? Maybe something like the following? > > (Untested, probably does not even build.) > > > > #define lockless_dereference(p) \ > > ({ \ > > typeof(*p) *_________p1 = ACCESS_ONCE(p); \ > > smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ > > _________p1; \ > > }) > > Hmm... Where would you prefer to put it? rcupdate.h? Good a place as any, I guess. Please see patch below. Left to myself, I would send this along for the next merge window, but please let me know if you would like it sooner. Thanx, Paul ------------------------------------------------------------------------ rcu: Provide counterpart to rcu_dereference() for non-RCU situations Although rcu_dereference() and friends can be used in situations where object lifetimes are being managed by something other than RCU, the resulting sparse and lockdep-RCU noise can be annoying. This commit therefore supplies a lockless_dereference(), which provides the protection for dereferences without the RCU-related debugging noise. Reported-by: Al Viro Signed-off-by: Paul E. McKenney diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 9575c2d403b5..ed4f5939a452 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -617,6 +617,21 @@ static inline void rcu_preempt_sleep_check(void) #define RCU_INITIALIZER(v) (typeof(*(v)) __force __rcu *)(v) /** + * lockless_dereference() - safely load a pointer for later dereference + * @p: The pointer to load + * + * Similar to rcu_dereference(), but for situations where the pointed-to + * object's lifetime is managed by something other than RCU. That + * "something other" might be reference counting or simple immortality. + */ +#define lockless_dereference(p) \ +({ \ + typeof(p) _________p1 = ACCESS_ONCE(p); \ + smp_read_barrier_depends(); /* Dependency order vs. p above. */ \ + (_________p1); \ +}) + +/** * rcu_assign_pointer() - assign to RCU-protected pointer * @p: pointer to assign to * @v: value to assign (publish) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/