Re: [PATCH] Update comment on find_task_by_pid_ns

["Serge E. Hallyn" <serue@us.ibm.com>]

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?

thanks,
-serge