linux-kernel - Re: [PATCH 46/51] trace, ring-buffer: Fix CPU hotplug callback registration

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Message-ID: <20140205184138.10f8cd15@gandalf.local.home>
Date:	Wed, 5 Feb 2014 18:41:38 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	paulus@...ba.org, oleg@...hat.com, rusty@...tcorp.com.au,
	peterz@...radead.org, tglx@...utronix.de,
	akpm@...ux-foundation.org, mingo@...nel.org,
	paulmck@...ux.vnet.ibm.com, tj@...nel.org, walken@...gle.com,
	ego@...ux.vnet.ibm.com, linux@....linux.org.uk,
	linux-kernel@...r.kernel.org,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 46/51] trace, ring-buffer: Fix CPU hotplug callback
 registration

On Thu, 06 Feb 2014 03:42:58 +0530
"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> wrote:

> Subsystems that want to register CPU hotplug callbacks, as well as perform
> initialization for the CPUs that are already online, often do it as shown
> below:
> 
> 	get_online_cpus();
> 
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
> 
> 	register_cpu_notifier(&foobar_cpu_notifier);
> 
> 	put_online_cpus();
> 
> This is wrong, since it is prone to ABBA deadlocks involving the
> cpu_add_remove_lock and the cpu_hotplug.lock (when running concurrently
> with CPU hotplug operations).
> 
> Instead, the correct and race-free way of performing the callback
> registration is:
> 
> 	cpu_maps_update_begin();
> 
> 	for_each_online_cpu(cpu)
> 		init_cpu(cpu);
> 
> 	/* Note the use of the double underscored version of the API */
> 	__register_cpu_notifier(&foobar_cpu_notifier);
> 
> 	cpu_maps_update_done();
> 
> 
> Fix the tracing ring-buffer code by using this latter form of callback
> registration.
> 
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
>

Acked-by: Steven Rostedt <rostedt@...dmis.org>

-- Steve
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