Re: [RFC PATCH V2 2/7] perf: Init/fini PMU specific data

[Peter Zijlstra]

On Mon, Jan 06, 2020 at 06:23:43AM -0800, Andi Kleen wrote:
> > > +	rcu_read_lock();
> > > +	for_each_process_thread(g, p) {
> > > +		mutex_lock(&p->perf_event_mutex);
> > > +		if (p->perf_ctx_data) {
> > > +			/*
> > > +			 * The perf_ctx_data for this thread may has been
> > > +			 * allocated by per-task event.
> > > +			 * Only update refcount for the case.
> > > +			 */
> > > +			refcount_inc(&p->perf_ctx_data->refcount);
> > > +			mutex_unlock(&p->perf_event_mutex);
> > > +			continue;
> > > +		}
> > > +
> > > +		if (pos < num_thread) {
> > > +			refcount_set(&data[pos]->refcount, 1);
> > > +			rcu_assign_pointer(p->perf_ctx_data, data[pos++]);
> > > +		} else {
> > > +			/*
> > > +			 * There may be some new threads created,
> > > +			 * when we allocate space.
> > > +			 * Track the number in nr_new_tasks.
> > > +			 */
> > > +			nr_new_tasks++;
> > > +		}
> > > +		mutex_unlock(&p->perf_event_mutex);
> > > +	}
> > > +	rcu_read_unlock();
> > > +
> > > +	raw_spin_unlock_irqrestore(&task_data_sys_wide_events_lock, flags);
> > 
> > Still NAK. That's some mightly broken code there.
> 
> Yes, Kan you cannot use a mutex (sleeping) inside rcu_read_lock().
> Can perf_event_mutex be a spin lock?

Or insize that raw_spin_lock. And last time I expressly said to not do
what whole tasklist iteration under a spinlock.