From: Erica Bugden <erica.bug...@linutronix.de> The hwlat tracer uses a kernel thread to measure latencies. The function that creates this kernel thread, start_kthread(), can be called when the tracer is initialized and when the tracer is explicitly enabled. start_kthread() does not check if there is an existing hwlat kernel thread and will create a new one each time it is called.
This causes the reference to the previous thread to be lost. Without the thread reference, the old kernel thread becomes unstoppable and continues to use CPU time even after the hwlat tracer has been disabled. This problem can be observed when a system is booted with tracing enabled and the hwlat tracer is configured like this: echo hwlat > current_tracer; echo 1 > tracing_on Add the missing check for an existing kernel thread in start_kthread() to prevent this problem. This function and the rest of the hwlat kernel thread setup and teardown are already serialized because they are called through the tracer core code with trace_type_lock held. [ Note, this only fixes the symptom. The real fix was not to call this function when tracing_on was already one. But this still makes the code more robust, so we'll add it. ] Link: http://lkml.kernel.org/r/1533120354-22923-1-git-send-email-erica.bug...@linutronix.de Signed-off-by: Erica Bugden <erica.bug...@linutronix.de> Signed-off-by: Steven Rostedt (VMware) <rost...@goodmis.org> --- kernel/trace/trace_hwlat.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c index d7c8e4ec3d9d..2d9d36dd5fe7 100644 --- a/kernel/trace/trace_hwlat.c +++ b/kernel/trace/trace_hwlat.c @@ -354,6 +354,9 @@ static int start_kthread(struct trace_array *tr) struct task_struct *kthread; int next_cpu; + if (hwlat_kthread) + return 0; + /* Just pick the first CPU on first iteration */ current_mask = &save_cpumask; get_online_cpus(); -- 2.18.0