linux-kernel - RE: volatile and atomic_t/spinlock

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Date:	Tue, 5 Jun 2007 11:38:27 -0700
From:	"Luck, Tony" <tony.luck@...el.com>
To:	"Heiko Carstens" <heiko.carstens@...ibm.com>,
	<linux-kernel@...r.kernel.org>, <linux-arch@...r.kernel.org>
Subject: RE: volatile and atomic_t/spinlock_t

> So is
>
>	while (__raw_spin_is_locked(&v));
>
> supposed to work? Or should that be 
>
>	while (__raw_spin_is_locked(&v))
>		cpu_relax();
>
> as well and all the volatiles can/should go away?

cpu_relax() is a really good idea in every spinloop on
hyper-threaded cores.  It lets the h/w know that we aren't
doing anything useful here, so resources and power can be
diverted to other threads sharing the core.

Avoiding the need for volatile or other compiler optimizer
defeating tricks is a side benefit.

-Tony
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