Davidlohr, Peter, I'll try to read this patch later, just one note.
On 12/05, Peter Zijlstra wrote:
>
> On Fri, Dec 02, 2016 at 06:18:39PM -0800, Davidlohr Bueso wrote:
> > @@ -102,8 +103,13 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
> > */
> > __this_cpu_dec(*sem->read_count);
> >
> > + rcu_read_lock();
> > + writer = rcu_dereference(sem->writer);
>
> Don't think this is correct, I think Oleg suggested using
> task_rcu_dereference(), which is a giant pile of magic.
Yes, but on a second thought task_rcu_dereference() won't really help,
but we can just use rcu_dereference().
> The problem is that task_struct isn't RCU protected as such.
Yes. But percpu_down_write() should not be used after exit_notify(), so we
can rely on rcu_read_lock(), release_task()->call_rcu(delayed_put_task_struct)
can't be called until an exiting task passes exit_notify().
But then we probably need WARN_ON(current->exit_state) in percpu_down_write().
And personally I think this change should add the new helpers, they can have
more users. Something like
struct xxx {
struct task_struct *task;
};
xxx_wake_up(struct xxx *xxx)
{
rcu_read_lock();
task = rcu_dereference(xxx->task);
if (task)
wake_up_process(task);
rcu_read_unlock();
}
#define xxx_wait_event(xxx, event) {
// comment to explain why
WARN_ON(current->exit_state);
xxx->task = current;
...
}
Oleg.
