linux-kernel - [PATCH 5.9 44/46] tracing: Remove WARN_ON in start

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Date:   Sun,  6 Dec 2020 12:17:52 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Vasily Averin <vvs@...tuozzo.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>
Subject: [PATCH 5.9 44/46] tracing: Remove WARN_ON in start_thread()

From: Vasily Averin <vvs@...tuozzo.com>

commit 310e3a4b5a4fc718a72201c1e4cf5c64ac6f5442 upstream.

This patch reverts commit 978defee11a5 ("tracing: Do a WARN_ON()
 if start_thread() in hwlat is called when thread exists")

.start hook can be legally called several times if according
tracer is stopped

screen window 1
[root@...alhost ~]# echo 1 > /sys/kernel/tracing/events/kmem/kfree/enable
[root@...alhost ~]# echo 1 > /sys/kernel/tracing/options/pause-on-trace
[root@...alhost ~]# less -F /sys/kernel/tracing/trace

screen window 2
[root@...alhost ~]# cat /sys/kernel/debug/tracing/tracing_on
0
[root@...alhost ~]# echo hwlat >  /sys/kernel/debug/tracing/current_tracer
[root@...alhost ~]# echo 1 > /sys/kernel/debug/tracing/tracing_on
[root@...alhost ~]# cat /sys/kernel/debug/tracing/tracing_on
0
[root@...alhost ~]# echo 2 > /sys/kernel/debug/tracing/tracing_on

triggers warning in dmesg:
WARNING: CPU: 3 PID: 1403 at kernel/trace/trace_hwlat.c:371 hwlat_tracer_start+0xc9/0xd0

Link: https://lkml.kernel.org/r/bd4d3e70-400d-9c82-7b73-a2d695e86b58@virtuozzo.com

Cc: Ingo Molnar <mingo@...hat.com>
Cc: stable@...r.kernel.org
Fixes: 978defee11a5 ("tracing: Do a WARN_ON() if start_thread() in hwlat is called when thread exists")
Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 kernel/trace/trace_hwlat.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -368,7 +368,7 @@ static int start_kthread(struct trace_ar
 	struct task_struct *kthread;
 	int next_cpu;
 
-	if (WARN_ON(hwlat_kthread))
+	if (hwlat_kthread)
 		return 0;
 
 	/* Just pick the first CPU on first iteration */