linux-kernel - Re: [PATCH] thinkpad-acpi: fix potential suspend blocking issue

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Message-ID: <20130306154420.GA7697@redhat.com>
Date:	Wed, 6 Mar 2013 16:44:20 +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: Re: [PATCH] thinkpad-acpi: fix potential suspend blocking issue

On 03/05, Henrique de Moraes Holschuh wrote:
>
> On Tue, 05 Mar 2013, Mandeep Singh Baines wrote:
> > This mutex seems wrong. Its held the entire time the kthread is
> > running. I think its used to synchronize on the exit of the kthread. A
> > completion would more appropriate in that case.
>
> From the top of the driver source:
>
> /* Acquired while the poller kthread is running, use to sync start/stop */
> static struct mutex hotkey_thread_mutex;

I simply can't understand what this "sync start/stop" means...

Ignoring hotkey_kthread(), the only user is

	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.

As for serializing with hotkey_poll_setup/etc, looks like this code relies
on hotkey_mutex.

So I think hotkey_thread_mutex can be simply removed?

Oleg.

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