Subject: sched: Move kprobes cleanup out of finish_task_switch()
From: Thomas Gleixner <[email-masked]>
Date: Tue, 28 Sep 2021 14:24:28 +0200

Doing cleanups in the tail of schedule() is a latency punishment for the
incoming task. The point of invoking kprobes_task_flush() for a dead task
is that the instances are returned and cannot leak when __schedule() is
kprobed.

Move it into the delayed cleanup.

Signed-off-by: Thomas Gleixner <[email-masked]>
Cc: Masami Hiramatsu <[email-masked]>
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
---
 kernel/exit.c       |    2 ++
 kernel/kprobes.c    |    8 ++++----
 kernel/sched/core.c |    6 ------
 3 files changed, 6 insertions(+), 10 deletions(-)

--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -64,6 +64,7 @@
 #include <linux/rcuwait.h>
 #include <linux/compat.h>
 #include <linux/io_uring.h>
+#include <linux/kprobes.h>
 
 #include <linux/uaccess.h>
 #include <asm/unistd.h>
@@ -168,6 +169,7 @@ static void delayed_put_task_struct(stru
 {
 	struct task_struct *tsk = container_of(rhp, struct task_struct, rcu);
 
+	kprobe_flush_task(tsk);
 	perf_event_delayed_put(tsk);
 	trace_sched_process_free(tsk);
 	put_task_struct(tsk);
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1250,10 +1250,10 @@ void kprobe_busy_end(void)
 }
 
 /*
- * This function is called from finish_task_switch when task tk becomes dead,
- * so that we can recycle any function-return probe instances associated
- * with this task. These left over instances represent probed functions
- * that have been called but will never return.
+ * This function is called from delayed_put_task_struct() when a task is
+ * dead and cleaned up to recycle any function-return probe instances
+ * associated with this task. These left over instances represent probed
+ * functions that have been called but will never return.
  */
 void kprobe_flush_task(struct task_struct *tk)
 {
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4845,12 +4845,6 @@ static struct rq *finish_task_switch(str
 		if (prev->sched_class->task_dead)
 			prev->sched_class->task_dead(prev);
 
-		/*
-		 * Remove function-return probe instances associated with this
-		 * task and put them back on the free list.
-		 */
-		kprobe_flush_task(prev);
-
 		/* Task is done with its stack. */
 		put_task_stack(prev);