Re: [PATCH RFC 1/9] hrtimer: Mark data-racy accesses to hrtimer_sleeper ->task field

[Paul E. McKenney]

On Tue, Aug 11, 2026 at 03:35:27PM +0200, Thomas Gleixner wrote:
> On Thu, Jul 30 2026 at 17:40, Paul E. McKenney wrote:
> >  static enum hrtimer_restart hrtimer_wakeup(struct hrtimer *timer)
> >  {
> >  	struct hrtimer_sleeper *t = container_of(timer, struct hrtimer_sleeper, timer);
> > -	struct task_struct *task = t->task;
> > +	struct task_struct *task = hrtimer_sleeper_task_get(t);
> >  
> > -	t->task = NULL;
> > +	hrtimer_sleeper_task_set(t, NULL);
> 
> To be honest, I find these helper functions more confusing that
> useful unless you make the task member private at the very end of the
> series once everything is converted over.

Good point, and thank you for the reminder.  As shown below?

> If not, then what's wrong with a plain READ/WRITE_ONCE()?

Nothing wrong with a good READ_ONCE() or WRITE_ONCE() as far as I am
concerned!  ;-)

							Thanx, Paul

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

commit cff6e1c78dbf97f50d19613266aa22d4b7588e95
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date:   Tue Aug 11 12:11:32 2026 -0700

    hrtimer: Mark the hrtimer_sleeper structure's ->task field __private
    
    The hrtimer_sleeper structure's ->task field is now used only by the
    hrtimer_sleeper_task_get() and hrtimer_sleeper_task_set() functions,
    and there is no reason for it to be directly accessed anywhere else.
    
    Therefore, mark this field __private and use ACCESS_PRIVATE() in
    hrtimer_sleeper_task_get() and hrtimer_sleeper_task_set().
    
    Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxx>
    Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index 3ad2ab0443af71..5c04423e8e900b 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -73,7 +73,7 @@ enum hrtimer_mode {
  */
 struct hrtimer_sleeper {
 	struct hrtimer timer;
-	struct task_struct *task;
+	struct task_struct __private *task;
 };
 
 static inline void hrtimer_set_expires(struct hrtimer *timer, ktime_t time)
@@ -352,11 +352,11 @@ extern int schedule_hrtimeout_range_clock(ktime_t *expires,
 extern int schedule_hrtimeout(ktime_t *expires, const enum hrtimer_mode mode);
 static inline struct task_struct *hrtimer_sleeper_task_get(struct hrtimer_sleeper *sl)
 {
-	return READ_ONCE(sl->task);
+	return READ_ONCE(ACCESS_PRIVATE(sl, task));
 }
 static inline void hrtimer_sleeper_task_set(struct hrtimer_sleeper *sl, struct task_struct *t)
 {
-	WRITE_ONCE(sl->task, t);
+	WRITE_ONCE(ACCESS_PRIVATE(sl, task), t);
 }
 
 /* Soft interrupt function to run the hrtimer queues: */