Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

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Hello,

On (11/25/10 12:02), Oleg Nesterov wrote:
> (another try, actually add Sergey)
>
Thank you.
 
> On 11/25, Peter Zijlstra wrote:
> >
> > On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote:
> > > --- a/kernel/posix-cpu-timers.c
> > > +++ b/kernel/posix-cpu-timers.c
> > > @@ -391,6 +391,7 @@ 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;
> > > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> > >  	} else {
> > >  		ret = -EINVAL;
> > >  	}
> > > +	rcu_read_unlock();
> > >  	read_unlock(&tasklist_lock);
> > >
> > >  	return ret;
> >
> > Do we still need the tasklist_lock in this case?
> 
> No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not
> completely trivial to remove it.
> 
> In particular, this patch is not exactly right, we can't trust
> thread_group_leader() without tasklist.
> 
> Sergey already sent the patch which removes tasklist from
> posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc
> Thomas queued it.
> 

You're right, Oleg.

Commit-ID:  c0deae8c9587419ab13874b74425ce2eb2e18508
Gitweb:     http://git.kernel.org/tip/c0deae8c9587419ab13874b74425ce2eb2e18508
Author:     Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
AuthorDate: Wed, 3 Nov 2010 18:52:56 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 10 Nov 2010 13:07:06 +0100

posix-cpu-timers: Rcu_read_lock/unlock protect find_task_by_vpid call

queued (15 days so far).


> > Also, why is that think complaining, surely the tasklist_lock pins any
> > and all PID objects?
> 
> The only problem is: if copy_process() fails, it does free_pid()
> lockless. This means, without rcu lock it is not safe to scan the
> rcu-protected lists.
> 
> We can change copy_process() (in fact I sent the patch several
> years ago), but everybody think that find_pid/etc should always
> take rcu_read_lock() instead. I tend to agree.
> 
> Oleg.
> 

	Sergey

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