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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Jun 13, 2017 at 07:23:08PM -0400, Steven Rostedt wrote:
> 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 *

Both reads of ->gp_flags are READ_ONCE(), so having seen the new value
in swait_event_interruptible(), this task/CPU cannot see the old value
from some later access.  You have to have accesses to two different
variables to require a memory barrier (at least assuming consistent use
of READ_ONCE(), WRITE_ONCE(), or equivalent).

> 				 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.

I -know- I have bugs now.  ;-)

But I don't believe this is one of them.  Or am I getting confused?

							Thanx, Paul