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Date: Wed, 10 Feb 2010 09:57:10 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>, oleg@redhat.com,
       tglx@linutronix.de, linux-kernel@vger.kernel.org,
       paulmck@linux.vnet.ibm.com, linux-security-module@vger.kernel.org
Subject: Re: [PATCH] Update comment on find_task_by_pid_ns
Message-Id: <20100210095710.c7b124f7.akpm@linux-foundation.org>
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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.