linux-kernel - Re: [patch] i386/x86_64: smp_call

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Date:	Fri, 9 Feb 2007 08:40:12 +0100
From:	Andi Kleen <ak@...e.de>
To:	Heiko Carstens <heiko.carstens@...ibm.com>
Cc:	Andrew Morton <akpm@...l.org>, Ingo Molnar <mingo@...e.hu>,
	Jan Glauber <jan.glauber@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] i386/x86_64: smp_call_function locking inconsistency

On Thursday 08 February 2007 21:32, Heiko Carstens wrote:
> On i386/x86_64 smp_call_function_single() takes call_lock with
> spin_lock_bh(). To me this would imply that it is legal to call
> smp_call_function_single() from softirq context.
> It's not since smp_call_function() takes call_lock with just
> spin_lock(). We can easily deadlock:
> 
> -> [process context]
> -> smp_call_function()
> -> spin_lock(&call_lock)
> -> IRQ -> do_softirq -> tasklet
> -> [softirq context]
> -> smp_call_function_single()
> -> spin_lock_bh(&call_lock)
> -> dead
> 
> So either all spin_lock_bh's should be converted to spin_lock,
> which would limit smp_call_function()/smp_call_function_single()
> to process context & irqs enabled.
> Or the spin_lock's could be converted to spin_lock_bh which would
> make it possible to call these two functions even if in softirq
> context. AFAICS this should be safe.

I'm not so sure. Perhaps drop _bh in both and stick a WARN_ON_ONCE in
to catch the cases? 

-Andi

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