linux-kernel - Re: [RFC][PATCH 0/4] Redesign cpu

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Message-Id: <20060827110143.663d8207.akpm@osdl.org>
Date:	Sun, 27 Aug 2006 11:01:43 -0700
From:	Andrew Morton <akpm@...l.org>
To:	dipankar@...ibm.com
Cc:	Linus Torvalds <torvalds@...l.org>, Dave Jones <davej@...hat.com>,
	ego@...ibm.com, rusty@...tcorp.com.au,
	linux-kernel@...r.kernel.org, arjan@...el.linux.com, mingo@...e.hu,
	vatsa@...ibm.com, ashok.raj@...el.com
Subject: Re: [RFC][PATCH 0/4] Redesign cpu_hotplug locking.

On Sun, 27 Aug 2006 23:19:46 +0530
Dipankar Sarma <dipankar@...ibm.com> wrote:

> I don't see why this
> is needed - 
> 
> + break;
> +
> + case CPU_DOWN_PREPARE:
> + 	mutex_lock(&workqueue_mutex);
> + 	break;
> +
> + case CPU_DOWN_FAILED:
> + 	mutex_unlock(&workqueue_mutex);
> 	break;
> 
> This seems like some implicit code locking to me. Why is it not
> sufficient to hold the lock in the CPU_DEAD code while walking
> the workqueues ?

?

We need to hold workqueue_mutex to protect the per-cpu workqueue resources
while cpu_online_map is changing and while per-cpu memory is being
allocated or freed.

Look at cpu_down() and mentally replace the
blocking_notifier_call_chain(CPU_DOWN_PREPARE) with
mutex_lock(workqueue_mutex), etc.  The __stop_machine_run() in there
modifies the (ie: potentially frees) the workqueue code's per-cpu memory. 
So we take that resource's lock while doing so.
-
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