linux-kernel - Re: [PATCH tip/sched/core] Add comments to aid in safer usage of swake

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Message-ID: <20170613192308.173dd86a@gandalf.local.home>
Date:   Tue, 13 Jun 2017 19:23:08 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Krister Johansen <kjlx@...pleofstupid.com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Paul Gortmaker <paul.gortmaker@...driver.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH tip/sched/core] Add comments to aid in safer usage of
 swake_up.

On Fri, 9 Jun 2017 05:45:54 -0700
"Paul E. McKenney" <paulmck@...ux.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