linux-kernel - Re: calling smp_call_function_many() with non-stable CPU mask

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Date:	Tue, 01 Feb 2011 02:44:52 -0600
From:	Milton Miller <miltonm@....com>
To:	Jan Beulich <JBeulich@...ell.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: calling smp_call_function_many() with non-stable CPU mask

> There are a couple of examples of smp_call_function_many() getting
> called with mm_cpumask() as the first argument. Since that mask
> generally can change while smp_call_function_many() is executing,
> it seems there might be a problem with the case where that mask
> becomes empty after the initial checks, but before the mask is made
> permanent (by copying into data->cpumask).
> 
> Shouldn't there be a check of data->refs being zero right after
> setting it (to avoid having csd_lock_wait() wait for a remote CPU
> to clear the lock flag, and to avoid adding the entry to
> call_function.queue)?
> 
> If that isn't considered necessary, is it then incorrect to pass
> in-flight CPU masks to smp_call_function_many() (and should
> this requirement then be documented somewhere, and the
> existing calls all be inspected for correctness)?
> 

Mike Galbraith just brought this up, and I supplied a patch, and even
a rediff on top of other changes:

http://lkml.indiana.edu/hypermail/linux/kernel/1101.3/02813.html
http://lkml.indiana.edu/hypermail/linux/kernel/1101.3/03172.html

http://lkml.indiana.edu/hypermail/linux/kernel/1102.0/00017.html


This doesn't address https://bugzilla.kernel.org/show_bug.cgi?id=23042 
which is x86 not expecting the mask to be cleared while its thinking
about the mask.

milton
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