Message-ID: <51AE667F.6030702@candelatech.com>
Date: Tue, 04 Jun 2013 15:13:19 -0700
From: Ben Greear <greearb@candelatech.com>
Organization: Candela Technologies
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130311 Thunderbird/17.0.4
MIME-Version: 1.0
To: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
CC: Rusty Russell <rusty@rustcorp.com.au>,
        Thomas Gleixner <tglx@linutronix.de>
Subject: Re: 3.9.x:  Possible race related to stop_machine leads to lockup.
References: <51AE5998.2060204@candelatech.com>
In-Reply-To: <51AE5998.2060204@candelatech.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1535
Lines: 47

On 06/04/2013 02:18 PM, Ben Greear wrote:
> I've been trying to figure out why I see the migration/* processes
> hang in a busy loop....
>
> While reading the stop_machine.c file, I think I might have an
> answer.
>
> The set_state() method sets the thread_ack to the current number
> of threads.  Each thread's state machine then decrements it down to
> zero where it bumps the state to the next level.  This lets each
> cpu stop in lock-step it seems.
>
> But, from what I can tell, the __stop_machine() method can
> (re)set the state to STOPMACHINE_PREPARE while the migration
> processes are in their loop.  That would explain why they sometimes
> loop forever.
>
> Does this make sense?

Err, no..that doesn't make sense.  'smdata' is on the stack.

More printk debugging makes it look like one thread just
never notices that smdata->state has been updated by another
thread.

There is this comment..maybe cpu_relax only does the chill out part
and we need something else to make sure smdata->state is freshly
read from the other CPU's cache?

		/* Chill out and ensure we re-read stopmachine_state. */
		cpu_relax();
		if (smdata->state != curstate) {

Gah..way out of my league :P

Ben


-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/