From: Sven Schnelle <svens@linux.ibm.com>
Date: Mon, 9 Dec 2019 09:03:12 +0100
Subject: s390/ftrace: fix endless recursion in function_graph tracer
Git-commit: 6feeee8efc53035c3195b02068b58ae947538aa4
Patch-mainline: v5.5-rc3
References: git-fixes bsc#1213912

The following sequence triggers a kernel stack overflow on s390x:

mount -t tracefs tracefs /sys/kernel/tracing
cd /sys/kernel/tracing
echo function_graph > current_tracer
[crash]

This is because preempt_count_{add,sub} are in the list of traced
functions, which can be demonstrated by:

echo preempt_count_add >set_ftrace_filter
echo function_graph > current_tracer
[crash]

The stack overflow happens because get_tod_clock_monotonic() gets called
by ftrace but itself calls preempt_{disable,enable}(), which leads to a
endless recursion. Fix this by using preempt_{disable,enable}_notrace().

Fixes: 011620688a71 ("s390/time: ensure get_clock_monotonic() returns monotonic values")
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Acked-by: Miroslav Franc <mfranc@suse.cz>
---
 arch/s390/include/asm/timex.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/s390/include/asm/timex.h b/arch/s390/include/asm/timex.h
index 6da8885251d6..670f14a228e5 100644
--- a/arch/s390/include/asm/timex.h
+++ b/arch/s390/include/asm/timex.h
@@ -194,9 +194,9 @@ static inline unsigned long long get_tod_clock_monotonic(void)
 {
 	unsigned long long tod;
 
-	preempt_disable();
+	preempt_disable_notrace();
 	tod = get_tod_clock() - *(unsigned long long *) &tod_clock_base[1];
-	preempt_enable();
+	preempt_enable_notrace();
 	return tod;
 }