Re: [patch 1/9] sys: Fix missing rcu protection for __task_cred() access

Thomas Gleixner
2009-12-11T13:53:31Z

On Fri, 11 Dec 2009, David Howells wrote:

> Thomas Gleixner <tglx@linutronix.de> wrote:
>
> > > > Or are there updates that are carried out without write-holding
> > > > tasklist_lock that I am missing?
> > >
> > > Yes, commit_creds() is called lockless.
> >
> > Right, and that's what the problem is. commit_creds(), which rcu frees
> > the old creds, does not take tasklist lock write lock.
>
> commit_creds() does not need to hold a write lock, because it is implicitly
> write-locked by only being permitted to run in the thread to which it is
> committing.
>
> I don't think commit_creds() needs to take the RCU read lock as no-one else
> can alter/delete the creds it is dealing with.

commit_cred() is not required to take anything, but the reader side
needs to take rcu_read_lock() when accessing __task_cred() of another
task as that task could update its own creds right at that point.

The point is that read_lock(tasklist_lock) is not sufficient for the
reader side.

Thanks,

tglx