linux-kernel - Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes to use function calls [POST 2]

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Message-ID: <20080819092754.GE28713@elte.hu>
Date:	Tue, 19 Aug 2008 11:27:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
	Andi Kleen <andi@...stfloor.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Jens Axboe <jens.axboe@...cle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 0 of 9] x86/smp function calls: convert x86 tlb flushes
	to use function calls [POST 2]


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> I think this might be a spurious "holding multiple locks in the same 
> class" bug.  All the queue locks are presumably in the same class, and 
> ipi_call_lock_irq() wants to hold them all to lock out any IPIs. 
> Spurious because this is the only place which holds more than one 
> queue lock, and it always locks 0->N.
> 
> I guess the fix is to use an outer lock and use spin_lock_nested() 
> (now that it exists).  Something along these lines?

this is not a good idea:

> +/* Hold queues_lock when taking more than one queue[].lock at once */
> +static DEFINE_SPINLOCK(queues_lock);

because it adds an artificial extra spinlock for no good reason and 
weakens the lock dependency checking as well.

Just add a lock class descriptor to each call_function_queue lock, so 
that lockdep can see that it's truly all in the correct order.

I.e. dont turn lockdep off artificially.

	Ingo
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