Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake_up.

Steven Rostedt
2017-06-13T23:23:13Z

On Fri, 9 Jun 2017 05:45:54 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> On Fri, Jun 09, 2017 at 09:19:57AM +0200, Peter Zijlstra wrote:
> > On Thu, Jun 08, 2017 at 08:25:46PM -0700, Krister Johansen wrote:
> > > The behavior of swake_up() differs from that of wake_up(), and from the
> > > swake_up() that came from RT linux. A memory barrier, or some other
> > > synchronization, is needed prior to a swake_up so that the waiter sees
> > > the condition set by the waker, and so that the waker does not see an
> > > empty wait list.
> >
> > Urgh.. let me stare at that. But it sounds like the wrong solution since
> > we wanted to keep the wait and swait APIs as close as possible.
>
> But don't they both need some sort of ordering, be it memory barriers or
> locking, to handle the case where the wait/swait doesn't actually sleep?
>

Looking at an RCU example, and assuming that ordering can move around
within a spin lock, and that changes can leak into a spin lock region
from both before and after. Could we have:

(looking at __call_rcu_core() and rcu_gp_kthread()

CPU0 CPU1
---- ----
__call_rcu_core() {

spin_lock(rnp_root)
need_wake = __rcu_start_gp() {
rcu_start_gp_advanced() {
gp_flags = FLAG_INIT
}
}

rcu_gp_kthread() {
swait_event_interruptible(wq,
gp_flags & FLAG_INIT) {
spin_lock(q->lock)

*fetch wq->task_list here! *

list_add(wq->task_list, q->task_list)
spin_unlock(q->lock);

*fetch old value of gp_flags here *

spin_unlock(rnp_root)

rcu_gp_kthread_wake() {
swake_up(wq) {
swait_active(wq) {
list_empty(wq->task_list)

} * return false *

if (condition) * false *
schedule();

Looks like a memory barrier is missing. Perhaps we should slap on into
swait_active()? I don't think it is wise to let users add there own, as
I think we currently have bugs now.

-- Steve