[PATCH 1/8] mm: introduce for_each_process_rcu and for_each_thread_rcu

[Ye Liu <ye.liu@linux.dev>]

From: Ye Liu <liuye@kylinos.cn>

Introduce for_each_process_rcu(), for_each_thread_rcu() and
for_each_process_thread_rcu() macros that acquire the RCU read lock
before the iteration starts and release it when the loop is left, so
that the RCU read-side critical section is scoped to the loop body
instead of an externally managed rcu_read_lock()/rcu_read_unlock()
pair.

Signed-off-by: Ye Liu <liuye@kylinos.cn>
---
 include/linux/sched/signal.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
index 584ae88b435e..fe0c41dc99a1 100644
--- a/include/linux/sched/signal.h
+++ b/include/linux/sched/signal.h
@@ -2,6 +2,7 @@
 #ifndef _LINUX_SCHED_SIGNAL_H
 #define _LINUX_SCHED_SIGNAL_H
 
+#include <linux/cleanup.h>
 #include <linux/rculist.h>
 #include <linux/signal.h>
 #include <linux/sched.h>
@@ -663,6 +664,24 @@ extern bool current_is_single_threaded(void);
 #define for_each_process_thread(p, t)	\
 	for_each_process(p) for_each_thread(p, t)
 
+/*
+ * RCU-internal variants: automatically acquire and release the RCU read
+ * lock around the iteration.  Equivalent to scoped_guard(rcu) combined
+ * with the respective non-_rcu variant.
+ */
+#define for_each_process_rcu(p) \
+	scoped_guard(rcu) \
+	for (p = &init_task ; (p = next_task(p)) != &init_task ; )
+
+#define for_each_thread_rcu(p, t) \
+	scoped_guard(rcu) \
+	__for_each_thread((p)->signal, t)
+
+/* Careful: this is a double loop, 'break' won't work as expected. */
+#define for_each_process_thread_rcu(p, t) \
+	scoped_guard(rcu) \
+	for_each_process(p) for_each_thread(p, t)
+
 typedef int (*proc_visitor)(struct task_struct *p, void *data);
 void walk_process_tree(struct task_struct *top, proc_visitor, void *);
 
-- 
2.25.1