linux-kernel - Re: [PATCH] smp.c: Quit unconditionally enabling irqs in on_each_cpu

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Date:	Fri, 9 Aug 2013 21:51:20 +0300
From:	Gilad Ben-Yossef <gilad@...yossef.com>
To:	David Daney <ddaney@...iumnetworks.com>
Cc:	Christoph Lameter <cl@...two.org>,
	David Daney <ddaney.cavm@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Chris Metcalf <cmetcalf@...era.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	David Daney <david.daney@...ium.com>
Subject: Re: [PATCH] smp.c: Quit unconditionally enabling irqs in on_each_cpu_mask().

On Thu, Aug 8, 2013 at 11:27 PM, David Daney <ddaney@...iumnetworks.com> wrote:
>
> On 08/08/2013 12:25 PM, Christoph Lameter wrote:
>>
>> On Thu, 8 Aug 2013, David Daney wrote:
>>
>>> I don't know of any bugs currently caused by this unconditional
>>> local_irq_enable(), but I want to use this function in MIPS/OCTEON
>>> early boot (when we have early_boot_irqs_disabled).  This also makes
>>> this function have similar semantics to on_each_cpu() which is good in
>>> itself.
>>
>>
>> smp_call_function_many() wants interrupts enabled.
>
>
> That's what the comments say, but it isn't actually true.
>
> The usage introduced by the patch is no different than the existing usage in on_each_cpu() 30 line up in the file.
>

Regardless of the question of how smp_call_function_many() should be
called, the IRQ disable/enable pair is actually there to make sure the
provided function runs on the current CPU at the same conditions as it
would get called via the IPI.

I would at least consider putting a test there to make sure IRQs
really are disabled when entering the function, otherwise the bugs
stemming from incorrect use can be tricky to catch.

Just my 0.00002 BTC
Gilad



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