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

Oleg Nesterov
2009-12-10T14:35:47Z

On 12/09, Paul E. McKenney wrote:
>
> On Thu, Dec 10, 2009 at 12:52:51AM -0000, Thomas Gleixner wrote:
> > commit c69e8d9 (CRED: Use RCU to access another task's creds and to
> > release a task's own creds) added non rcu_read_lock() protected access
> > to task creds of the target task in set_prio_one().
> >
> > The comment above the function says:
> > * - the caller must hold the RCU read lock
> >
> > The calling code in sys_setpriority does read_lock(&tasklist_lock) but
> > not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
> > With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
> > interrupt when they see no read side critical section.
> >
> > There is another instance of __task_cred() in sys_setpriority() itself
> > which is equally unprotected.
> >
> > Wrap the whole code section into a rcu read side critical section to
> > fix this quick and dirty.
> >
> > Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
> > crusade.
>
> OK, I will bite... Don't the corresponding updates write-hold
> tasklist_lock? If so, then the fact that the code below is read-holding
> tasklist_lock would prevent any of the data from changing, which would
> remove the need to do the rcu_read_lock().
>
> Or are there updates that are carried out without write-holding
> tasklist_lock that I am missing?

Yes, commit_creds() is called lockless.

Or I misunderstood the question/patch...

Oleg.