linux-kernel - Re: [PATCH] ipc/sem.c: Update/correct memory barriers.

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Message-ID: <20150301132829.GB20691@redhat.com>
Date:	Sun, 1 Mar 2015 14:28:29 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
	Kirill Tkhai <ktkhai@...allels.com>,
	Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] ipc/sem.c: Update/correct memory barriers.

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.

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