linux-kernel - [PATCH 0/1] thinkpad-acpi: kill hotkey_thread

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Date:	Thu, 7 Mar 2013 18:53:05 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:	Mandeep Singh Baines <msb@...omium.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-acpi@...r.kernel.org, ibm-acpi@....eng.br,
	ibm-acpi-devel@...ts.sourceforge.net,
	platform-driver-x86@...r.kernel.org, Aaron Lu <aaron.lu@...el.com>,
	Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: [PATCH 0/1] thinkpad-acpi: kill hotkey_thread_mutex

On 03/06, Henrique de Moraes Holschuh wrote:
>
> On Wed, 06 Mar 2013, Oleg Nesterov wrote:
> >
> > 	static void hotkey_poll_stop_sync(void)
> > 	{
> > 		if (tpacpi_hotkey_task) {
> > 			kthread_stop(tpacpi_hotkey_task);
> > 			tpacpi_hotkey_task = NULL;
> > 			mutex_lock(&hotkey_thread_mutex);
> > 			/* at this point, the thread did exit */
> > 			mutex_unlock(&hotkey_thread_mutex);
> > 		}
> > 	}
> >
> > And I simply do not understand the comment. This thread has already exited
> > when kthread_stop() returns (OK, it can be running do_exit() paths but this
> > doesn't matter). So this mutex_lock() buys nothing afaics.
>
> It was added due to an oops, waaaaay back then.  If it is not needed
> anymore, and there is zero chance of the kthread still being active when
> hotkey_poll_stop_sync() ends, hotkey_thread_mutex can be simply removed.

Well, there could be another bug. Say, hotkey_poll_stop_sync() can block
on hotkey_thread_mutex if another thread was started. But at first glance
this can't happen (hotkey_mutex), and even _if_ it can this needs another
fix.

> Looks like it, if the current semanthics of ktread_stop() are syncronous.

IIRC, it always was... But at least currently it is certainly syncronous.
kthread_stop(t) does wait_for_completion(t->vfork_done), complete(vfork_done)
can't happen unless this task calls do_exit().

Hmm. I just noticed that the recent changes in kthread_stop() are not correct...
But this is offtopic and doesn't affect thinkpad_acpi.c, I'll write another
email later.

So, what do you think about (UNTESTED) 1/1 ?

Oleg.

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