linux-kernel - Re: irq lock inversion

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Message-ID: <4AF3B9BD.9050300@kernel.org>
Date:	Fri, 06 Nov 2009 14:53:01 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Jiri Kosina <jkosina@...e.cz>
CC:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Yinghai Lu <yhlu.kernel@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>, cl@...ux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: irq lock inversion

Hello, Jiri.

Jiri Kosina wrote:
> Hmm, I wonder why I don't see this lockdep warning myself with 
> head on 1836d9592, even though I have 
> 
> 	CONFIG_PROVE_LOCKING=y
> 	CONFIG_TRACE_IRQFLAGS=y
> 
> ... ?

You need pcpu_mem_free() hit vfree() to trigger the warning by
allocating a lot of small percpu areas so that allocation map inside a
chunk becomes larger than 4k and then get extended once more.

> Anyway, how about something like this? (I can't verify myself that it even 
> fixes the warning, as I don't see it for some odd reason)
> 
> From: Jiri Kosina <jkosina@...e.cz>
> Subject: lockdep: avoid false positives about irq-safety
> 
> Commit 403a91b1 ("percpu: allow pcpu_alloc() to be called
> with IRQs off") introduced this warning:
> 
> 	=========================================================
> 	[ INFO: possible irq lock inversion dependency detected ]
> 	2.6.32-rc5-tip-04815-g12f0f93-dirty #745
> 	---------------------------------------------------------
> 	hub 1-3:1.0: state 7 ports 2 chg 0000 evt 0004
> 	ksoftirqd/65/199 just changed the state of lock:
> 	 (pcpu_lock){..-...}, at: [<ffffffff81130e04>] free_percpu+0x38/0x104
> 	but this lock took another, SOFTIRQ-unsafe lock in the past:
> 	 (vmap_area_lock){+.+...}
> 
> 	and interrupts could create inverse lock ordering between them.
> 
> This warning is bogus -- sched_init() is being called very early with IRQs
> disabled, and the irqsave/restore code paths in pcpu_alloc() are only for early
> init. The path can never be called from irq context once the early init
> finishes. Rationale for this is explained in changelog of the commit mentioned
> above.
> 
> This problem can be encountered generally in any other early code running
> with IRQs off and using irqsave/irqrestore.
> 
> Reported-by: Yinghai Lu <yhlu.kernel@...il.com>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>

Looks good to me.  Ingo, what do you think?

Thanks.

-- 
tejun
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