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From: Andrew Morton <akpm@linux-foundation.org>
To: Oleg Nesterov <oleg@tv-sign.ru>
Cc: ebiederm@xmission.com, mingo@elte.hu, paulmck@linux.vnet.ibm.com,
	xemul@openvz.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU
Date: Tue, 29 Jan 2008 15:08:36 -0800	[thread overview]
Message-ID: <20080129150836.7fd994a2.akpm@linux-foundation.org> (raw)
In-Reply-To: <20080129164019.GA2060@tv-sign.ru>

On Tue, 29 Jan 2008 19:40:19 +0300
Oleg Nesterov <oleg@tv-sign.ru> wrote:

> With CONFIG_PREEMPT_RCU read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> but find_pid_ns()->hlist_for_each_entry_rcu() should be safe under tasklist.
> 
> Usually it is, detach_pid() is always called under write_lock(tasklist_lock),
> but copy_process() calls free_pid() lockless.
> 
> "#ifdef CONFIG_PREEMPT_RCU" is added mostly as documentation, perhaps it is
> too ugly and should be removed.
> 
> Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
> 
> --- MM/kernel/fork.c~PR_RCU	2008-01-27 17:09:47.000000000 +0300
> +++ MM/kernel/fork.c	2008-01-29 19:23:44.000000000 +0300
> @@ -1335,8 +1335,19 @@ static struct task_struct *copy_process(
>  	return p;
>  
>  bad_fork_free_pid:
> -	if (pid != &init_struct_pid)
> +	if (pid != &init_struct_pid) {
> +#ifdef CONFIG_PREEMPT_RCU
> +		/*
> +		 * read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
> +		 * make sure find_pid() is safe under read_lock(tasklist).
> +		 */
> +		write_lock_irq(&tasklist_lock);
> +#endif
>  		free_pid(pid);
> +#ifdef CONFIG_PREEMPT_RCU
> +		write_unlock_irq(&tasklist_lock);
> +#endif
> +	}
>  bad_fork_cleanup_namespaces:
>  	exit_task_namespaces(p);
>  bad_fork_cleanup_keys:

My attempt to understand this change timed out.

kernel/pid.c is full of global but undocumented functions.  What are the
locking requirements for free_pid()?  free_pid_ns()?  If it's just
caller-must-hold-rcu_read_lock() then why not use rcu_read_lock() here?

If the locking is "caller must hold write_lock_irq(tasklist_lock) then the
sole relevant comment in there (in free_pid()) is wrong.

Guys, more maintainable code please?

  parent reply	other threads:[~2008-01-29 23:09 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-01-29 16:40 Oleg Nesterov
2008-01-29 23:02 ` Andrew Morton
2008-01-30 14:17   ` Peter Zijlstra
2008-01-31 13:32   ` Ingo Molnar
2008-01-29 23:08 ` Andrew Morton [this message]
2008-01-30  2:16   ` Eric W. Biederman
2008-01-30  4:56     ` Paul E. McKenney
2008-01-30  3:24 ` Eric W. Biederman
2008-01-30  5:00   ` Paul E. McKenney
2008-01-30  9:20     ` Eric W. Biederman
2008-01-30  9:48       ` Oleg Nesterov
2008-01-30  9:30   ` Oleg Nesterov
2008-01-30 18:28     ` Eric W. Biederman
2008-01-31  9:31       ` Oleg Nesterov
2009-12-14  2:15 Tetsuo Handa

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