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

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Message-ID: <20150301132232.GA20691@redhat.com>
Date:	Sun, 1 Mar 2015 14:22:32 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Manfred Spraul <manfred@...orfullife.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 02/28, 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.

I agree, so perhaps it should be named smp_acquire_after_unlock_wait ?
even if it is actually stronger than "acquire"...

To me "control_barrier" looks a bit confusing. I think this helper should
be only used after spin_unlock_wait() or spin_is_locked/unlocked(). In this
case it is clear that this "barrier" pairs with release semantics of
spin_unlock(). And we use it because we want to serialize with that unlock,
as if we are taking this lock.

But I won't insist.

Oleg.

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