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 12:59:32 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	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>,
	ak@...e.de, 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 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.

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.
--
vda
-
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