Re: [PATCH] Update comment on find_task_by_pid_ns

Paul E. McKenney
2010-02-10T20:31:00Z

On Wed, Feb 10, 2010 at 02:18:34PM -0600, Serge E. Hallyn wrote:
> Quoting Thomas Gleixner (tglx@linutronix.de):
> > On Wed, 10 Feb 2010, Andrew Morton wrote:
> > > On Wed, 10 Feb 2010 10:30:33 -0600 "Serge E. Hallyn" <serue@us.ibm.com> wrote:
> > >
> > > > Quoting Andrew Morton (akpm@linux-foundation.org):
> > > > > On Tue, 9 Feb 2010 06:42:45 +0900
> > > > > Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> wrote:
> > > > >
> > > > > > OK. I updated description.
> > > > > >
> > > > > > As of 2.6.32 , below users are missing rcu_read_lock().
> > > > > >
> > > > > > Users missing rcu_read_lock() when calling find_task_by_vpid():
> > > > > >
> > > > > > SYSCALL_DEFINE3(ioprio_set) in fs/ioprio.c
> > > > > > SYSCALL_DEFINE2(ioprio_get) in fs/ioprio.c
> > > > > > cap_get_target_pid() in kernel/capability.c
> > > > >
> > > > > Actually, cap_get_target_pid() uses rcu_read_lock() and doesn't take
> > > > > tasklist_lock.
> > > >
> > > > Hmm - is that in -mm? In my copy here it takes read_lock(&tasklist_lock)
> > >
> > > yup. It got changed in linux-next.
> > >
> > > > And I'll admit I'm a bit confused as to the current state of things:
> > > > do I understand correctly that we now need to take both the tasklist_lock
> > > > and rcu_read_lock? (Presumably only for read_lock()?)
> > >
> > > Beats me. We need to protect both the pid->task_struct lookup data
> > > structures (during the lookup) and protect the resulting task_struct
> > > while the caller is playing with it. It's unclear whether
> > > rcu_read_lock() suffices for both purposes.
> >
> > The rcu_read_lock section is sufficient. task_struct can not go away
> > before the rcu_read_unlock()
>
> But, more generally, it used to be the case that a spinlock (or
> rwlock) would imply an rcu read cycle, but now it no longer does,
> do I understand that right?

You are correct, with CONFIG_PREEMPT_RCU, acquiring a lock does -not-
imply an RCU read-side critical section. And acquiring a lock does
-not- imply any sort of RCU read-side critical section in -rt kernels
in any case.

Thanx, Paul