[PATCH v5 2/3] sched: Provide copy_user_cpus_mask() to copy out user mask

[Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

Since accessing the content of the user_cpus_ptr requires lock protection
to ensure its validity, provide a helper function copy_user_cpus_mask()
to facilitate its reading.

Suggested-by: Peter Zijlstra <peterz-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>
Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
 include/linux/sched.h |  1 +
 kernel/sched/core.c   | 19 +++++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index e7b2f8a5c711..f2b0340c094e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1830,6 +1830,7 @@ extern int task_can_attach(struct task_struct *p, const struct cpumask *cs_effec
 extern void do_set_cpus_allowed(struct task_struct *p, const struct cpumask *new_mask);
 extern int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask);
 extern int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src, int node);
+extern struct cpumask *copy_user_cpus_mask(struct task_struct *p, struct cpumask *user_mask);
 extern void release_user_cpus_ptr(struct task_struct *p);
 extern int dl_task_check_affinity(struct task_struct *p, const struct cpumask *mask);
 extern void force_compatible_cpus_allowed_ptr(struct task_struct *p);
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 03053eebb22e..a0987784913e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2618,6 +2618,25 @@ void release_user_cpus_ptr(struct task_struct *p)
 	kfree(clear_user_cpus_ptr(p));
 }
 
+/*
+ * Return the copied mask pointer or NULL if user mask not available.
+ * Acquiring pi_lock for read access protection.
+ */
+struct cpumask *copy_user_cpus_mask(struct task_struct *p,
+				    struct cpumask *user_mask)
+{
+	struct cpumask *mask = NULL;
+	unsigned long flags;
+
+	raw_spin_lock_irqsave(&p->pi_lock, flags);
+	if (p->user_cpus_ptr) {
+		cpumask_copy(user_mask, p->user_cpus_ptr);
+		mask = user_mask;
+	}
+	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+	return mask;
+}
+
 /*
  * This function is wildly self concurrent; here be dragons.
  *
-- 
2.31.1