On Wed, Aug 18, 2021 at 11:42:28AM +0100, Will Deacon wrote:

As for your other suggestion:

quoted

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c

@@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked
 	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
 	const struct cpumask *cpu_valid_mask = cpu_active_mask;
 	bool kthread = p->flags & PF_KTHREAD;
+	struct cpumask *user_mask = NULL;
 	unsigned int dest_cpu;
 	int ret = 0;
 

@@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked
 	__do_set_cpus_allowed(p, new_mask, flags);
 
 	if (flags & SCA_USER)
-		release_user_cpus_ptr(p);
+		swap(user_mask, p->user_cpus_ptr);
+
+	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
+
+	kfree(user_mask);
 
-	return affine_move_task(rq, p, rf, dest_cpu, flags);
+	return ret;
 
 out:
 	task_rq_unlock(rq, p, rf);

@@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s
 		return;
 
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
-	release_user_cpus_ptr(p);
+	p->user_cpus_ptr = NULL;
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+	kfree(mask);

I think the idea looks good, but perhaps we could wrap things up a bit:

/* Comment about why this is useful with RT */
static cpumask_t *clear_user_cpus_ptr(struct task_struct *p)
{
	struct cpumask *user_mask = NULL;

	swap(user_mask, p->user_cpus_ptr);
	return user_mask;
}

void release_user_cpus_ptr(struct task_struct *p)
{
	kfree(clear_user_cpus_ptr(p));
}

Then just use clear_user_cpus_ptr() in sched/core.c where we know what
we're doing (well, at least one of us does!).

OK, I'll go make it like that.

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