linux-kernel - Re: [RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.

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Message-ID: <20071017021112.GA15895@in.ibm.com>
Date:	Wed, 17 Oct 2007 07:41:12 +0530
From:	Dipankar Sarma <dipankar@...ibm.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Gautham R Shenoy <ego@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Rusty Russel <rusty@...tcorp.com.au>,
	Oleg Nesterov <oleg@...sign.ru>, Ingo Molnar <mingo@...e.hu>,
	Paul E McKenney <paulmck@...ibm.com>
Subject: Re: [RFC PATCH 0/4] Refcount Based Cpu-Hotplug Revisit.

On Tue, Oct 16, 2007 at 10:20:37AM -0700, Linus Torvalds wrote:
> On Tue, 16 Oct 2007, Gautham R Shenoy wrote:
> 
> Well, afaik, the patch series is fairly clean, and I'm obviously perfectly 
> happy with the approach, so I have no objections. 
> 
> But it looks buggy. This:
> 
> 	+static void cpu_hotplug_begin(void)
> 	+{
> 	+       mutex_lock(&cpu_hotplug.lock);
> 	+       cpu_hotplug.active_writer = current;
> 	+       while (cpu_hotplug.refcount) {
> 	+               mutex_unlock(&cpu_hotplug.lock);
> 	+               wait_for_completion(&cpu_hotplug.readers_done);
> 	+               mutex_lock(&cpu_hotplug.lock);
> 	+       }
> 	+
> 	+}
> 
> drops the cpu_hotplug.lock, which - as far as I can see - means that 
> another process can come in and do the same, and mess up the 
> "active_writer" thing. The oerson that actually *gets* the lock may not be 
> the same one that has "active_writer" set to itself. No? Am I missing 
> something.

Unless I am reading the patch wrongly, it seems cpu_hotplug_begin() is called 
while holding the cpu_add_remove_lock mutex. So, another CPU cannot come in
and do the same until _cpu_down() is over.

Thanks
Dipankar
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