linux-kernel - Re: [PATCH] sched: Move level 'again' after get_cpu() in sched

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Date:	Tue, 12 Jan 2010 11:50:38 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: Move level 'again' after get_cpu() in
 sched_exec.

On Tue, 2010-01-12 at 16:42 +0600, Rakib Mullick wrote:
> sched: move level again after get_cpu().
> 
>  We can get this_cpu by calling get_cpu() once and it remains
> same. We don't need to call it again.

Yes we do... get_cpu() needs to be paired with put_cpu() and they
disable/enable preemption.

Since there's an unconditional put_cpu() before the goto again, we need
to do the get_cpu().

Plenty of kernel debug options would have informed you of this if you'd
tried running it.

> Signed-off-by: Rakib Mullick <rakib.mullick@...il.com>
> ---
> 
> --- linus/kernel/sched.c	2010-01-06 20:11:10.000000000 +0600
> +++ rakib/kernel/sched.c	2010-01-12 16:01:54.000000000 +0600
> @@ -3143,8 +3143,8 @@ void sched_exec(void)
>  	unsigned long flags;
>  	struct rq *rq;
> 
> -again:
>  	this_cpu = get_cpu();
> +again:
>  	dest_cpu = select_task_rq(p, SD_BALANCE_EXEC, 0);
>  	if (dest_cpu == this_cpu) {
>  		put_cpu();

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