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

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Date:	Sun, 1 Mar 2015 20:16:46 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Manfred Spraul <manfred@...orfullife.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, 1vier1@....de,
	Peter Zijlstra <peterz@...radead.org>,
	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

Manfred,

I leave this to you and Paul/Peter, but...

On 03/01, Manfred Spraul wrote:
>
> +/*
> + * spin_unlock_wait() and !spin_is_locked() are not memory barriers, they
> + * are only control barriers, thus a memory barrier is required if the
> + * operation should act as an acquire memory barrier, i.e. if it should
> + * pair with the release memory barrier from the spin_unlock() that released
> + * the spinlock.
> + * smp_rmb() is sufficient, as writes cannot pass the implicit control barrier.
> + */
> +#ifndef smp_acquire__after_spin_unlock_wait
> +#define smp_acquire__after_spin_unlock_wait()	smp_rmb()
> +#endif
> +#ifndef smp_acquire__after_spin_is_unlocked
> +#define smp_acquire__after_spin_is_unlocked()	smp_rmb()
> +#endif

But spin_unlock_wait() and spin_is_locked() is the "same thing" when it
comes to serialization with spin_unlock()... Not sure we need 2 helpers.

But I won't argue of course.

Oleg.

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