linux-kernel - Re: irq lock inversion

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Date:	Fri, 06 Nov 2009 17:24:16 +0900
From:	Tejun Heo <tj@...nel.org>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Nick Piggin <npiggin@...e.de>, Jiri Kosina <jkosina@...e.cz>,
	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, Ingo.

Ingo Molnar wrote:
> I havent looked deeply but at first sight i'm not 100% sure that even 
> the lock dance hack is safe - doesnt vfree() do TLB flushes, which must 
> be done with irqs enabled in general? If yes, then the whole notion of 
> using the allocator from irqs-off sections is wrong and the flags 
> save/restore is misguided (or at least incomplete).

The only place where any v*() call is nested under pcpu_lock is in the
alloc path, specifically pcpu_extend_area_map() ends up calling
vfree().  pcpu_free() path which can be called from irq context never
calls any vmalloc function directly.  The reclaiming is deferred to a
work.  Breaking the single nesting completely decouples the two locks
and nobody would be calling vfree() with irq disabled, so I don't
think there will be any problem.

Thanks.

-- 
tejun
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