linux-kernel - Re: [PATCH] Relay CPU Hotplug support

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Date:	Sat, 23 Dec 2006 01:31:13 +0530
From:	Gautham R Shenoy <ego@...ibm.com>
To:	Andrew Morton <akpm@...l.org>
Cc:	ego@...ibm.com, Oleg Nesterov <oleg@...sign.ru>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	linux-kernel@...r.kernel.org, David Wilder <dwilder@...ibm.com>,
	Tom Zanussi <zanussi@...ibm.com>,
	Ingo Molnar <mingo@...hat.com>,
	Greg Kroah-Hartman <gregkh@...e.de>,
	Christoph Hellwig <hch@...radead.org>, ltt-dev@...fik.org,
	systemtap@...rces.redhat.com,
	Douglas Niehaus <niehaus@...s.ku.edu>,
	"Martin J. Bligh" <mbligh@...igh.org>,
	Thomas Gleixner <tglx@...utronix.de>, kiran@...lex86.org,
	venkatesh.pallipadi@...el.com, dipankar@...ibm.com,
	vatsa@...ibm.com, torvalds@...l.org, davej@...hat.com
Subject: Re: [PATCH] Relay CPU Hotplug support

On Fri, Dec 22, 2006 at 02:44:58AM -0800, Andrew Morton wrote:
> On Fri, 22 Dec 2006 16:07:24 +0530
> Gautham R Shenoy <ego@...ibm.com> wrote:
> 
> > While we are at this per-subsystem cpuhotplug "locking", here's a
> > proposal that might put an end to the workqueue deadlock woes.
> 
> Oleg is working on some patches which will permit us to cancel or wait upon
> a particular work_struct, rather than upon all pending work_structs.
>

Oh! I was refering to the other set of workqueue deadlock woes. The
ones caused when subsystems (like cpufreq) try to create/destroy
workqueue from the cpuhotplug callback path. 

Creation/destruction of workqueue requires us to take workqueue_mutex,
which would have already been taken during CPU_LOCK_ACQUIRE.

More often than not, the cpu hotplug protection that we need
is while accessing either cpu_online_map OR one of it's persubsystem
mirrors like policy->cpus. 
So it makes more sense to have all the persubsystem 
mutexes held only during the cpu-hotplug operation (i.e stop_machine_run
and __cpu_up) and release them immediately after sending notifications to
update the persubsystem online_cpu map.

Thanks and Regards
gautham.
-- 
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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