linux-kernel - Re: [PATCH 2/2] posix-cpu-timers: remove tasklist

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Date:	Fri, 12 Jun 2009 09:11:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Stanislaw Gruszka <sgruszka@...hat.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] posix-cpu-timers: remove tasklist_lock where we can

On Fri, 2009-06-12 at 08:49 +0200, Stanislaw Gruszka wrote:
> tasklist_lock is not needed to protect find_task_by_vpid() nor
> thread_group_leader() nor same_thread_group() , use rcu_read_lock() instead.

Aside from not being strictly needed, one can sometimes require a
tasklist_lock reader to ensure nothing changes, however since the code
is racy in that regard anyway, this looks good.

Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>

> Signed-off-by: Stanislaw Gruszka <sgruszka@...hat.com>
> ---
>  kernel/posix-cpu-timers.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index bece7c0..1d9cdf3 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/posix-cpu-timers.c
> @@ -37,13 +37,13 @@ static int check_clock(const clockid_t which_clock)
>  	if (pid == 0)
>  		return 0;
>  
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	p = find_task_by_vpid(pid);
>  	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
>  		   same_thread_group(p, current) : thread_group_leader(p))) {
>  		error = -EINVAL;
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_lock();
>  
>  	return error;
>  }
> @@ -398,7 +398,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  	new_timer->it.cpu.incr.sched = 0;
>  	new_timer->it.cpu.expires.sched = 0;
>  
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
>  		if (pid == 0) {
>  			p = current;
> @@ -422,7 +422,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  	} else {
>  		ret = -EINVAL;
>  	}
> -	read_unlock(&tasklist_lock);
> +	rcu_read_unlock();
>  
>  	return ret;
>  }

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