linux-kernel - Re: [PATCH 2/4] rcu/tasks: Handle new PF

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Message-ID: <e4896e0b-eacc-45a2-a7a8-de2280a51ecc@paulmck-laptop>
Date:   Tue, 31 Oct 2023 11:12:01 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Neeraj Upadhyay <neeraj.upadhyay@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Uladzislau Rezki <urezki@...il.com>, rcu <rcu@...r.kernel.org>,
        Zqiang <qiang.zhang1211@...il.com>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>, matz@...e.de,
        ubizjak@...il.com
Subject: Re: [PATCH 2/4] rcu/tasks: Handle new PF_IDLE semantics

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);
 }