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

[Daniel Bristot de Oliveira]

On 6/18/21 9:00 PM, Steven Rostedt wrote:
> On Fri, 18 Jun 2021 12:45:03 -0400
> Steven Rostedt <rostedt@xxxxxxxxxxx> 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.
>

Yep! I tried to take the trace_type_lock here, and got the lockdep info about
this problem.

> 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.

So, it will not wait for the kworker to run?

-- Daniel

> 
> -- Steve
>