linux-kernel - RE: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing: convert bts-tracer mutex to a spinlock

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Message-ID: <928CFBE8E7CB0040959E56B4EA41A77E4A4C1FE3@irsmsx504.ger.corp.intel.com>
Date:	Wed, 25 Feb 2009 11:35:32 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing:
	convert bts-tracer mutex to a spinlock

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Wednesday, February 25, 2009 12:11 PM
>To: Metzger, Markus T
>Cc: hpa@...or.com; mingo@...hat.com; tglx@...utronix.de; linux-kernel@...r.kernel.org; linux-tip-
>commits@...r.kernel.org


>> And I assume you like the spinlock better than the
>> get/put_online_cpus(), as well.
>
>yeah - and get/put_online_cpus is sleepable too, so it doesnt
>really help unless i'm missing something ...

I suggested to use get/put_online_cpus() instead of the lock.

The hotplug code waits until the cpu_hotplug.refcount is zero
and it holds the cpu_hotplug.lock during hotplug operations
(see cpu_hotplug_begin/done()).

In turn, get_online_cpus() needs to grab the cpu_hotplug.lock
to increment the cpu_hotplug.refcount.

Thus, we will use the cpu_hotplug.lock instead of our own lock.

regards,
markus.

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