linux-kernel - Re: [PATCH] softlockup: remove timestamp checking from hung

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Message-ID: <20090209192956.GA7332@google.com>
Date:	Mon, 9 Feb 2009 11:29:56 -0800
From:	Mandeep Singh Baines <msb@...gle.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, rientjes@...gle.com,
	mbligh@...gle.com, thockin@...gle.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] softlockup: remove timestamp checking from hung_task

Frederic Weisbecker (fweisbec@...il.com) wrote:
> BTW, here is a small fixlet on top of your patch about what I commented concerning
> the tasks than weren't yet scheduled once:
> 
> --
> From e7120e424b031978e482b5fe311d90916ffb8b7e Mon Sep 17 00:00:00 2001
> From: Frederic Weisbecker <fweisbec@...il.com>
> Date: Sat, 7 Feb 2009 17:45:12 +0100
> Subject: [PATCH] softlockup: ensure the task has been scheduled once
> 
> When we check if the task has been scheduled since the last scan, we might
> have a race condition if the task has been inserted on the task list but not
> yet scheduled once. So we just add a small check to ensure it has been switched
> in at least one time to avoid false positive.
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> ---
>  kernel/hung_task.c |    6 +++++-
>  1 files changed, 5 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 4a10756..7f57a71 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -72,7 +72,11 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>  {
>  	unsigned long switch_count = t->nvcsw + t->nivcsw;
>  
> -	if (t->flags & PF_FROZEN)
> +	/*
> +	 * Ensure the task is not frozen and that it has been scheduled
> +	 * at least once.
> +	 */
> +	if (t->flags & PF_FROZEN || !switch_count)
>  		return;
>  
>  	if (switch_count != t->last_switch_count) {
> -- 
> 1.6.1
> 

Good catch! I would change the description though. The race is a little
more subtle than your current description. For a newly forked process, the race
can occur if check_hung_task() processes the task in the time between the task
changing its state to UNINTERRUPTIBLE and the the scheduler updating the
switch_count (nivcsw or nvcsw).

One minor change to the  code comment. You want to ensure that the task has
context switched at least once, NOT that the task has been scheduled at least
once.

If you re-send the patch with the fixed comment and description, I'll
ack it.
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