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

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Message-ID: <20101108131958.GD2580@linux.vnet.ibm.com>
Date:	Mon, 8 Nov 2010 05:19:58 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	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 Mon, Nov 08, 2010 at 12:28:17PM +0200, Sergey Senozhatsky wrote:
> On (11/07/10 19:01), Paul E. McKenney wrote:
> > On Mon, Nov 08, 2010 at 07:04:43AM +0900, Tetsuo Handa wrote:
> > > Hello.
> > > 
> > > Paul E. McKenney wrote:
> > > > > Users missing rcu_read_lock() when calling find_task_by_vpid():
> > > > > 
> > > > >   check_clock() in kernel/posix-cpu-timers.c
> > > > 
> > > > 	This one has read_lock(&tasklist_lock).
> > > > 
> > > Excuse me. Holding tasklist_lock lock does not help.
> > > We must call rcu_read_lock() explicitly.
> > > That's why 9728e5d6 "kernel/pid.c: update comment on find_task_by_pid_ns" was made.
> > 
> > OK, good point, there are a few more kernels of unpopped corn here.
> >
> 
> Hello,
> I prepared a patch for posix-cpu-timers. 
> 
> [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call
> Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> 
> http://lkml.org/lkml/2010/11/3/257

Ah, very good, thank you!!!

							Thanx, Paul

> 	Sergey
> 
>  
> > > I think there are users who needlessly call read_lock(&tasklist_lock)
> > > when they can use rcu_read_lock() instead.
> > > But I don't know when to use read_lock(&tasklist_lock).
> > > 
> > > If read_lock(&tasklist_lock) is needed only when we want to access
> > > the "struct task_struct" after rcu_read_unlock(), maybe it is cleaner to
> > > use a helper like
> > > 
> > > struct task_struct *find_task_and_get(pid_t pid)
> > > {
> > > 	struct task_struct *task;
> > > 	read_lock(&tasklist_lock);
> > > 	rcu_read_lock();
> > > 	task = find_task_by_vpid(pid);
> > > 	rcu_read_unlock();
> > > 	if (task)
> > > 		get_task_struct(task);
> > > 	read_unlock(&tasklist_lock);
> > > 	return task;
> > > }
> > > 
> > > and hide tasklist_lock.
> > 
> > This makes a lot of sense to me!  That said, most of the current
> > open-coded variants of your find_task_and_get() seem to have the
> > rcu_read_unlock() after the get_task_struct() rather than before.  But I
> > don't claim to understand the locking design of this part of the kernel
> > well enough to say which is the best approach.
> > 
> > So, either way, will you be submitting the patches for this?
> > 
> > 							Thanx, Paul
> > 


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