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

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Message-ID: <20150228233401.GM15405@linux.vnet.ibm.com>
Date:	Sat, 28 Feb 2015 15:34:02 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Manfred Spraul <manfred@...orfullife.com>,
	Oleg Nesterov <oleg@...hat.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 10:45:33PM +0100, Peter Zijlstra wrote:
> 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?

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?

							Thanx, Paul

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