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

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Date:	Sat, 30 Oct 2010 14:02:44 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	sergey.senozhatsky@...il.com, viro@...iv.linux.org.uk,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, mingo@...e.hu
Subject: Re: [PATCH] rcu_read_lock/unlock protect find_task_by_vpid call

On Sat, Oct 30, 2010 at 10:14:23PM +0900, Tetsuo Handa wrote:
> Sergey Senozhatsky wrote:
> > On (10/29/10 13:16), Paul E. McKenney wrote:
> > > Interesting...
> > > 
> > > The task-list lock is read-held at this point, which should mean that
> > > the PID mapping cannot change.  The lockdep_tasklist_lock_is_held()
> > > function does lockdep_is_held(&tasklist_lock), which must therefore
> > > only be checking for write-holding the lock.  The fix would be to
> > > make lockdep_tasklist_lock_is_held() check for either read-holding or
> > > write-holding tasklist lock.
> > > 
> > > Or is there some subtle reason that read-holding the tasklist lock is
> > > not sufficient?
> 
> This was discussed in the thread at http://kerneltrap.org/mailarchive/linux-kernel/2009/12/10/4517520 .
> 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.
> 
> And now the patch that adds
> 
> 	rcu_lockdep_assert(rcu_read_lock_held());
> 
> was merged in accordance with that comment.
> Therefore, I thing below change is not good.
> 
> > Should it be changed to (let's say)
> > 
> > struct task_struct *find_task_by_pid_ns(pid_t nr, struct pid_namespace *ns)
> > {
> > -	rcu_lockdep_assert(rcu_read_lock_held());
> > +	rcu_lockdep_assert(rcu_read_lock_held() || lockdep_tasklist_lock_is_held());
> > 	return pid_task(find_pid_ns(nr, ns), PIDTYPE_PID);
> > }

So we should remove the lockdep_tasklist_lock_is_held() and then
apply Sergey's patch, correct?

							Thanx, Paul
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