netdev - Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init

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Date:	Fri, 24 Aug 2007 14:07:25 +0200
From:	Andi Kleen <ak@...e.de>
To:	Denys Vlasenko <vda.linux@...glemail.com>
Cc:	Satyam Sharma <satyam@...radead.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Chris Snook <csnook@...hat.com>, clameter@....com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-arch@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	netdev@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	davem@...emloft.net, schwidefsky@...ibm.com, wensong@...ux-vs.org,
	horms@...ge.net.au, wjiang@...ilience.com, cfriesen@...tel.com,
	zlynx@....org, rpjday@...dspring.com, jesper.juhl@...il.com,
	segher@...nel.crashing.org
Subject: Re: [PATCH] i386: Fix a couple busy loops in mach_wakecpu.h:wait_for_init_deassert()

On Friday 24 August 2007 13:59:32 Denys Vlasenko wrote:
> On Thursday 16 August 2007 01:39, Satyam Sharma wrote:
> >
> >  static inline void wait_for_init_deassert(atomic_t *deassert)
> >  {
> > -	while (!atomic_read(deassert));
> > +	while (!atomic_read(deassert))
> > +		cpu_relax();
> >  	return;
> >  }
> 
> For less-than-briliant people like me, it's totally non-obvious that
> cpu_relax() is needed for correctness here, not just to make P4 happy.

I find it also non obvious. It would be really better to have a barrier
or equivalent (volatile or variable clobber) in the atomic_read()
 
> IOW: "atomic_read" name quite unambiguously means "I will read
> this variable from main memory". Which is not true and creates
> potential for confusion and bugs.

Agreed.

-Andi
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