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Date: Tue, 2 Nov 2010 16:31:37 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
cc: Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>, Ingo Molnar <mingo@elte.hu>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect
 find_task_by_vpid call
In-Reply-To: <20101102135821.GA5964@swordfish.minsk.epam.com>
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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.

Thanks,

	tglx
 
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
> 
> ---
> 
> diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
> index 6842eeb..855bc53 100644
> --- a/kernel/posix-cpu-timers.c
> +++ b/kernel/posix-cpu-timers.c
> @@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
>  		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;
>  	}
> +	rcu_read_unlock();
>  	read_unlock(&tasklist_lock);
>  
>  	return error;
> @@ -395,17 +397,21 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
>  		if (pid == 0) {
>  			p = current;
>  		} else {
> +			rcu_read_lock();
>  			p = find_task_by_vpid(pid);
>  			if (p && !same_thread_group(p, current))
>  				p = NULL;
> +			rcu_read_unlock();
>  		}
>  	} else {
>  		if (pid == 0) {
>  			p = current->group_leader;
>  		} else {
> +			rcu_read_lock();
>  			p = find_task_by_vpid(pid);
>  			if (p && !thread_group_leader(p))
>  				p = NULL;
> +			rcu_read_unlock();
>  		}
>  	}
>  	new_timer->it.cpu.task = p;
>