[PATCH v2 11/12] sched: More notrace

[Peter Zijlstra <peterz@infradead.org>]

[-- Attachment #1: peterz-kill-preempt_active-11.patch --]
[-- Type: text/plain, Size: 2159 bytes --]

preempt_schedule_common() is marked notrace, but it does not use
_notrace() preempt_count functions and __schedule() is also not marked
notrace, which means that its perfectly possible to end up in the
tracer from preempt_schedule_common().

Steve says:

| Yep, there's some history to this. This was originally the issue that
| caused function tracing to go into infinite recursion. But now we have
| preempt_schedule_notrace(), which is used by the function tracer, and
| that function must not be traced till preemption is disabled.
|
| Now if function tracing is running and we take an interrupt when
| NEED_RESCHED is set, it calls
|
|   preempt_schedule_common() (not traced)
|
| But then that calls preempt_disable() (traced)
|
| function tracer calls preempt_disable_notrace() followed by
| preempt_enable_notrace() which will see NEED_RESCHED set, and it will
| call preempt_schedule_notrace(), which stops the recursion, but
| still calls __schedule() here, and that means when we return, we call
| the __schedule() from preempt_schedule_common().
|
| That said, I prefer this patch. Preemption is disabled before calling
| __schedule(), and we get rid of a one round recursion with the
| scheduler.

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 kernel/sched/core.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -3057,7 +3057,7 @@ pick_next_task(struct rq *rq, struct tas
  *
  * WARNING: must be called with preemption disabled!
  */
-static void __sched __schedule(bool preempt)
+static void __sched notrace __schedule(bool preempt)
 {
 	struct task_struct *prev, *next;
 	unsigned long *switch_count;
@@ -3203,9 +3203,9 @@ void __sched schedule_preempt_disabled(v
 static void __sched notrace preempt_schedule_common(void)
 {
 	do {
-		preempt_disable();
+		preempt_disable_notrace();
 		__schedule(true);
-		sched_preempt_enable_no_resched();
+		preempt_enable_no_resched_notrace();
 
 		/*
 		 * Check again in case we missed a preemption opportunity