linux-kernel - Re: volatile and atomic_t/spinlock

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Message-ID: <20070605221736.GA9315@osiris.ibm.com>
Date:	Wed, 6 Jun 2007 00:17:36 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	"Luck, Tony" <tony.luck@...el.com>
Cc:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: volatile and atomic_t/spinlock_t

On Tue, Jun 05, 2007 at 11:38:27AM -0700, Luck, Tony wrote:
> > 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.

Currently it is already that it has to be

	while (__raw_spin_is_locked(&v))
		cpu_relax();

Just like in __raw_spin_unlock_wait(). Oh well, I should have
checked more before posting...
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