Re: [PATCH V4 05/12] trace/hwlat: Support hotplug operations

[Steven Rostedt <rostedt@goodmis.org>]

On Fri, 18 Jun 2021 12:45:03 -0400
Steven Rostedt <rostedt@goodmis.org> wrote:

> > +/*
> > + * hwlat_cpu_init - CPU hotplug online callback function
> > + */
> > +static int hwlat_cpu_init(unsigned int cpu)
> > +{
> > +	struct trace_array *tr = hwlat_trace;
> > +  
> 
> You need to take the trace_types_lock here, between testing the
> hwlat_busy and starting the threads. Otherwise, between the two, the
> hwlat tracer could be turned off while a CPU is coming on line, and
> then you just started a per cpu thread, while the hwlat tracer is not
> enabled.

And of course, because get_online_cpus() is called within
trace_types_lock, doing this check is going to cause a lock inversion.

The only thing I could think of is to wake up a worker thread to do the
work. That is, this just wakes the worker thread, then the worker grabs
the trace_types_lock, iterates through the cpu mask of expect running
threads, and then starts or kills them depending on the hwlat_busy
value.

-- Steve

> 
> > +	if (!hwlat_busy)
> > +		return 0;
> > +
> > +	if (!cpumask_test_cpu(cpu, tr->tracing_cpumask))
> > +		return 0;
> > +
> > +	return start_cpu_kthread(cpu);
> > +}