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

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Message-ID: <20200106143138.GN2844@hirez.programming.kicks-ass.net>
Date:   Mon, 6 Jan 2020 15:31:38 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     kan.liang@...ux.intel.com, mingo@...hat.com, acme@...nel.org,
        tglx@...utronix.de, bp@...en8.de, linux-kernel@...r.kernel.org,
        eranian@...gle.com, alexey.budankov@...ux.intel.com,
        vitaly.slobodskoy@...el.com
Subject: Re: [RFC PATCH V2 2/7] perf: Init/fini PMU specific data

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.