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

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Date:	Sat, 28 Feb 2015 22:45:33 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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 Sat, Feb 28, 2015 at 09:36:15PM +0100, Manfred Spraul wrote:
> +/*
> + * Place this after a control barrier (such as e.g. a spin_unlock_wait())
> + * to ensure that reads cannot be moved ahead of the control_barrier.
> + * Writes do not need a barrier, they are not speculated and thus cannot
> + * pass the control barrier.
> + */
> +#ifndef smp_mb__after_control_barrier
> +#define smp_mb__after_control_barrier()	smp_rmb()
> +#endif

Sorry to go bike shedding again; but should we call this:

smp_acquire__after_control_barrier() ?

The thing is; its not a full MB because:

 - stores might actually creep into it; while the control dependency
   guarantees stores will not creep out, nothing is stopping them from
   getting in;

 - its not transitive, and our MB is defined to be so.

Oleg, Paul?
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