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

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Message-Id: <4D47D21E020000780002F9FB@vpn.id2.novell.com>
Date:	Tue, 01 Feb 2011 08:27:58 +0000
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	<linux-kernel@...r.kernel.org>
Cc:	"Michael Galbraith" <MGalbraith@...ell.com>
Subject: 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)?

Thanks, Jan

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