Re: [PATCH] Update comment on find_task_by_pid_ns

Andrew Morton
2010-02-10T17:57:43Z

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.