linux-kernel - Re: fix bogus hotplug cpu warning

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Message-ID: <20070828090709.GA24578@elte.hu>
Date:	Tue, 28 Aug 2007 11:07:09 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Hugh Dickins <hugh@...itas.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: fix bogus hotplug cpu warning


this commit:

 commit d243769d3f83b318813a04a9592bb7cfedc6c280
 Author: Hugh Dickins <hugh@...itas.com>
 Date:   Mon Aug 27 16:06:19 2007 +0100

    fix bogus hotplug cpu warning

    Fix bogus DEBUG_PREEMPT warning on x86_64, when cpu brought online 
    after bootup: current_is_keventd is right to note its use of 
    smp_processor_id is preempt-safe, but should use 
    raw_smp_processor_id to avoid the warning.

-       int cpu = smp_processor_id();   /* preempt-safe: keventd is per-cpu */
+       int cpu = raw_smp_processor_id(); /* preempt-safe: keventd is per-cpu */

is incorrect. smp_processor_id() will _not_ warn about true per-cpu 
tasks:

        /*
         * Kernel threads bound to a single CPU can safely use
         * smp_processor_id():
         */
        this_mask = cpumask_of_cpu(this_cpu);

        if (cpus_equal(current->cpus_allowed, this_mask))
                goto out;

so if this check triggered for the cpu-offline code it's because keventd 
was _not_ per-cpu (perhaps because the cpu offline code broke its 
affinity?). So your patch hides a real bug.

	Ingo
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