From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Date: Sun, 1 Mar 2015 14:28:29 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Manfred Spraul <manfred@colorfullife.com>,
	LKML <linux-kernel@vger.kernel.org>, 1vier1@web.de,
	Kirill Tkhai <ktkhai@parallels.com>,
	Ingo Molnar <mingo@redhat.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>, stable@vger.kernel.org
Subject: Re: [PATCH] ipc/sem.c: Update/correct memory barriers.
Message-ID: <20150301132829.GB20691@redhat.com>
References: <1425155775-23909-1-git-send-email-manfred@colorfullife.com> <20150228214533.GY5029@twins.programming.kicks-ass.net> <20150228233401.GM15405@linux.vnet.ibm.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20150228233401.GM15405@linux.vnet.ibm.com>
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <stable.vger.kernel.org>

On 02/28, Paul E. McKenney wrote:
>
> The idea is that this would become a no-op on x86, s390, sparc &c, an isb
> instruction on ARM, an isync instruction on Power, and I cannot remember
> what on Itanium?  The other idea being to provide read-to-read control
> ordering in addition to the current read-to-write control ordering?

To me, the only purpose is documentation. Let's look at task_work_run()

		/*
		 * Synchronize with task_work_cancel(). It can't remove
		 * the first entry == work, cmpxchg(task_works) should
		 * fail, but it can play with *work and other entries.
		 */
		raw_spin_unlock_wait(&task->pi_lock);
		smp_mb();

It doesn't need the full mb() too. But rmb() will look very confusing
without a fat comment. So I think that it would be nice to write this
comment once and put it into the new helper.

Oleg.