linux-kernel - Re: [PATCH] sched: correct testing need_resched in mutex_spin_on

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Date:	Tue, 07 Jun 2011 15:47:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Hillf Danton <dhillf@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH] sched: correct testing need_resched in
 mutex_spin_on_owner()

On Tue, 2011-06-07 at 21:41 +0800, Hillf Danton wrote:
> It is suppose to check the owner task that is not absolutly running on the
> local CPU, 

Oh, why do you think so?

> and if NEED_RESCHED is happenly set on the current task of local
> CPU, we get incorrect result.

Only if your above assumption holds, which it doesn't. It explicitly
checks to see if _this_ cpu needs a resched while spinning, if so it
bails the spinning and calls schedule in the lock slow path.

If the owner cpu reschedules, owner will leave the rq and
owner_running() will return false, also breaking the loop.

> 
> Signed-off-by: Hillf Danton <dhillf@...il.com>
> ---
>  kernel/sched.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index fd18f39..3ea64fe 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -4326,7 +4326,7 @@ int mutex_spin_on_owner(struct mutex *lock,
> struct task_struct *owner)
>  		return 0;
> 
>  	while (owner_running(lock, owner)) {
> -		if (need_resched())
> +		if (test_tsk_need_resched(owner))
>  			return 0;
> 
>  		arch_mutex_cpu_relax();

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