linux-kernel - Re: [RFC PATCH] ARM: keep __my_cpu_offset consistent with generic one

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Message-ID: <20130307034454.GE25137@mudshark.cambridge.arm.com>
Date:	Thu, 7 Mar 2013 03:44:55 +0000
From:	Will Deacon <will.deacon@....com>
To:	Ming Lei <tom.leiming@...il.com>
Cc:	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	Nicolas Pitre <nico@...aro.org>,
	Russell King <rmk+kernel@....linux.org.uk>
Subject: Re: [RFC PATCH] ARM: keep __my_cpu_offset consistent with generic one

Hello,

On Thu, Mar 07, 2013 at 02:47:48AM +0000, Ming Lei wrote:
> Commit 14318efb(ARM: 7587/1: implement optimized percpu variable access)
> introduces arm's __my_cpu_offset to optimize percpu vaiable access,
> which really works well on hackbench test, but may cause __my_cpu_offset
> to return garbage value before it is initialized in cpu_init() called
> by setup_arch, so accessing a percpu variable before setup_arch may cause
> kernel hang. But the generic__my_cpu_offset always returns zero before
> percpu area is brought up.
> 
> So the patch tries to clear __my_cpu_offset on boot CPU early
> to avoid boot hang.
> 
> At least now percpu variable is accessed by lockdep before
> setup_arch(), and enabling CONFIG_LOCK_STAT or CONFIG_DEBUG_LOCKDEP
> can trigger kernel hang.

For the record, can you include a backtrace or something please? The
description makes it sounds like a caller bug, so it would be good to
document a valid user of per-cpu before cpu_init().

Will
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