From: Vasily Averin vvs@virtuozzo.com

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

screen window 2 [root@localhost ~]# cat /sys/kernel/debug/tracing/tracing_on 0 [root@localhost ~]# echo hwlat > /sys/kernel/debug/tracing/current_tracer [root@localhost ~]# echo 1 > /sys/kernel/debug/tracing/tracing_on [root@localhost ~]# cat /sys/kernel/debug/tracing/tracing_on 0 [root@localhost ~]# 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@redhat.com Cc: stable@vger.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@virtuozzo.com Signed-off-by: Steven Rostedt (VMware) rostedt@goodmis.org --- kernel/trace/trace_hwlat.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c index c9ad5c6fbaad..d071fc271eef 100644 --- a/kernel/trace/trace_hwlat.c +++ b/kernel/trace/trace_hwlat.c @@ -368,7 +368,7 @@ static int start_kthread(struct trace_array *tr) 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 */