linux-kernel - RE: [patch 1/6] x86, ftrace, hw-branch-tracer: support hotplug cpus

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E4A4C1E88@irsmsx504.ger.corp.intel.com>
Date:	Wed, 25 Feb 2009 09:01:50 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Nathan Lynch <ntl@...ox.com>, Ingo Molnar <mingo@...e.hu>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"hpa@...or.com" <hpa@...or.com>,
	"markus.t.metzger@...il.com" <markus.t.metzger@...il.com>,
	"srostedt@...hat.com" <srostedt@...hat.com>
Subject: RE: [patch 1/6] x86, ftrace, hw-branch-tracer: support hotplug cpus

>-----Original Message-----
>From: Nathan Lynch [mailto:ntl@...ox.com]
>Sent: Tuesday, February 24, 2009 10:46 PM
>To: Ingo Molnar
>Cc: Metzger, Markus T; linux-kernel@...r.kernel.org; tglx@...utronix.de; hpa@...or.com;
>markus.t.metzger@...il.com; srostedt@...hat.com
>Subject: Re: [patch 1/6] x86, ftrace, hw-branch-tracer: support hotplug cpus
>
>On Tue, 24 Feb 2009 20:36:22 +0100
>Ingo Molnar <mingo@...e.hu> wrote:
>
>>
>> * Markus Metzger <markus.t.metzger@...el.com> wrote:
>>
>> > Support hotplug cpus.
>>
>> > +static int __cpuinit bts_hotcpu_handler(struct notifier_block *nfb,
>> > +				     unsigned long action, void *hcpu)
>> > +{
>> > +	unsigned int cpu = (unsigned long)hcpu;
>> > +
>> > +	mutex_lock(&bts_tracer_mutex);
>>
>> This is buggy: CPU hotplug handlers must not sleep (they are
>> called with irqs disabled)
>
>I don't think that is the case; the cpu notifier chains are "raw", not
>atomic, and the cpu hotplug core doesn't disable irqs while processing
>the chains afaict.  And there are several current examples of cpu
>hotplug callbacks performing sleeping operations (slab and slub,
>page_alloc, probably many others).

The hotplug code uses mutexes around raw_notifier_call_chain() calls in
all but one case:
  For CPU_STARTING_FROZEN, the caller has interrupts disabled.

I guess I could check the notification reason before grabbing the lock
in the handler.

regards,
markus.
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