linux-kernel - Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by

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Date:	Tue, 2 Nov 2010 17:04:33 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect
 find_task_by_vpid call

Just added Oleg to the cc-list

On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:

> On (11/02/10 16:31), Thomas Gleixner wrote:
> > On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:
> > 
> > > Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for
> > > find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns.
> > > Add rcu_read_lock/rcu_read_unlock to call find_task_by_vpid.
> > > 
> > > Tetsuo Handa wrote:
> > > 
> > > Quoting from one of posts in that thead
> > > http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388
> > > 
> > > | Usually tasklist gives enough protection, but if copy_process() fails
> > > | it calls free_pid() lockless and does call_rcu(delayed_put_pid().   
> > > | This means, without rcu lock find_pid_ns() can't scan the hash table
> > > | safely.
> > 
> > We can remove the tasklist_lock while at it. rcu_read_lock is enough.
> > 
> 
> Please kindly review.
> 
> ---
> 
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index 6842eeb..96fe2a3 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_unlock();
>  
>  	return error;
>  }
> @@ -390,7 +390,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  
>  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
>  
> -	read_lock(&tasklist_lock);
> +	rcu_read_lock();
>  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
>  		if (pid == 0) {
>  			p = current;
> @@ -414,7 +414,7 @@ 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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