linux-kernel - Re: [PATCH] Fix CPU spinlock lockups on secondary CPU bringup

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Message-ID: <1308768408.2268.2.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Wed, 22 Jun 2011 20:46:48 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-omap@...r.kernel.org
Subject: Re: [PATCH] Fix CPU spinlock lockups on secondary CPU bringup

Le mercredi 22 juin 2011 à 11:55 +0100, Russell King - ARM Linux a
écrit :
> From: Russell King <rmk+kernel@....linux.org.uk>
> 
> Secondary CPU bringup typically calls calibrate_delay() during its
> initialization.  However, calibrate_delay() modifies a global variable
> (loops_per_jiffy) used for udelay() and __delay().
> 
> A side effect of 71c696b1 (calibrate: extract fall-back calculation
> into own helper) introduced in the 2.6.39 merge window means that we
> end up with a substantial period where loops_per_jiffy is zero.  This
> causes the spinlock debugging code to malfunction:
...

>  
> +	loops_per_jiffy = lpj;
>  	printed = true;
>  }

To be 100% safe, I would use

	ACCESS_ONCE(loops_per_jiffy) = lpj;

But I assume no current gcc would be that stupid ;)



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