Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics

[Paul E. McKenney]

On Tue, Oct 31, 2023 at 04:20:33PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 31, 2023 at 07:24:13AM -0700, Paul E. McKenney wrote:
> 
> > So, at least until GCC catches up to clang's code generation, I take it
> > that you don't want WRITE_ONCE() for that ->nvcsw increment.  Thoughts on
> > ->on_rq?
> 
> I've not done the patch yet, but I suspect those would be fine, those
> are straight up stores, hard to get wrong (famous last words).

Assuming that the reads are already either marked with READ_ONCE() or
are under appropriate locks, my immediate thought would be something
like the all-too-lightly tested patch shown below.

The ASSERT_EXCLUSIVE_WRITER() causes KCSAN to complain if there is a
concurrent store of any kind to the location.

Of course, please feel free to ignore.  Thoughts?

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 81885748871d..aeace19ad7f5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2124,12 +2124,14 @@ void activate_task(struct rq *rq, struct task_struct *p, int flags)
 
 	enqueue_task(rq, p, flags);
 
-	p->on_rq = TASK_ON_RQ_QUEUED;
+	WRITE_ONCE(p->on_rq, TASK_ON_RQ_QUEUED);
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 }
 
 void deactivate_task(struct rq *rq, struct task_struct *p, int flags)
 {
-	p->on_rq = (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING;
+	WRITE_ONCE(p->on_rq, (flags & DEQUEUE_SLEEP) ? 0 : TASK_ON_RQ_MIGRATING);
+	ASSERT_EXCLUSIVE_WRITER(p->on_rq);
 
 	dequeue_task(rq, p, flags);
 }