Date:   Wed, 17 May 2017 07:55:20 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [RFC][PATCH 0/5] perf/tracing/cpuhotplug: Fix locking order

On Wed, May 17, 2017 at 12:40:10PM +0200, Peter Zijlstra wrote:
> On Tue, May 16, 2017 at 07:27:42AM -0700, Paul E. McKenney wrote:
> > On Tue, May 16, 2017 at 05:46:06AM -0700, Paul E. McKenney wrote:
> 
> > > Something like this, yes.  Maybe even exactly like this.  ;-)
> > 
> > Ah, one thing I forgot...  If you are avoiding use of get_online_cpus(),
> > you usually also have to be very careful about how you use things like
> > cpu_online() and cpu_is_offline.
> 
> OK, so I think I got it wrong there. This hunk should close any race
> between perf_pmu_register() and hotplug by tracking a global state
> protected by pmus_lock. Thereby insuring the cpuctx->online state gets
> initialized right.

Looks much better!

> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -8993,6 +8993,8 @@ static int pmu_dev_alloc(struct pmu *pmu
>  static struct lock_class_key cpuctx_mutex;
>  static struct lock_class_key cpuctx_lock;
> 
> +static cpumask_var_t perf_online_mask;
> +
>  int perf_pmu_register(struct pmu *pmu, const char *name, int type)
>  {
>  	int cpu, ret;
> @@ -9056,7 +9058,7 @@ int perf_pmu_register(struct pmu *pmu, c
>  		lockdep_set_class(&cpuctx->ctx.mutex, &cpuctx_mutex);
>  		lockdep_set_class(&cpuctx->ctx.lock, &cpuctx_lock);
>  		cpuctx->ctx.pmu = pmu;
> -		cpuctx->online = cpu_online(cpu);
> +		cpuctx->online = cpumask_test_cpu(cpu, perf_online_mask);
> 
>  		__perf_mux_hrtimer_init(cpuctx, cpu);
>  	}
> @@ -10952,6 +10954,8 @@ static void __init perf_event_init_all_c
>  	struct swevent_htable *swhash;
>  	int cpu;
> 
> +	zalloc_cpumask_var(&perf_online_mask, GFP_KERNEL);
> +
>  	for_each_possible_cpu(cpu) {
>  		swhash = &per_cpu(swevent_htable, cpu);
>  		mutex_init(&swhash->hlist_mutex);
> @@ -11011,6 +11015,7 @@ static void perf_event_exit_cpu_context(
>  		cpuctx->online = 0;
>  		mutex_unlock(&ctx->mutex);
>  	}
> +	cpumask_clear_cpu(cpu, perf_online_mask);
>  	mutex_unlock(&pmus_lock);
>  }
>  #else
> @@ -11028,6 +11033,7 @@ int perf_event_init_cpu(unsigned int cpu
>  	perf_swevent_init_cpu(cpu);
> 
>  	mutex_lock(&pmus_lock);
> +	cpumask_set_cpu(cpu, perf_online_mask);
>  	list_for_each_entry(pmu, &pmus, entry) {
>  		cpuctx = per_cpu_ptr(pmu->pmu_cpu_context, cpu);
>  		ctx = &cpuctx->ctx;
> 
> > > > ---
> > > > --- a/kernel/events/core.c
> > > > +++ b/kernel/events/core.c
> > > > @@ -8997,7 +8997,6 @@ int perf_pmu_register(struct pmu *pmu, c
> > > >  {
> > > >  	int cpu, ret;
> > > > 
> > > > -	get_online_cpus();
> > > >  	mutex_lock(&pmus_lock);
> > > >  	ret = -ENOMEM;
> > > >  	pmu->pmu_disable_count = alloc_percpu(int);
> > > 
> > > There is usually also some state check in here somewhere for the CPU
> > > being offline from a perf perspective.  Such a check might already exist,
> > > but I must plead ignorance of perf.
> 
> This just allocates per-cpu storage, that is per definition on the
> possible mask and unrelated to the online mask.

Got it!

> > > > @@ -9093,7 +9092,6 @@ int perf_pmu_register(struct pmu *pmu, c
> > > >  	ret = 0;
> > > >  unlock:
> > > >  	mutex_unlock(&pmus_lock);
> > > > -	put_online_cpus();
> > > > 
> > > >  	return ret;
> > > > 
> > > > @@ -11002,10 +11000,9 @@ static void perf_event_exit_cpu_context(
> > > >  	struct perf_cpu_context *cpuctx;
> > > >  	struct perf_event_context *ctx;
> > > >  	struct pmu *pmu;
> > > > -	int idx;
> > > > 
> > > > -	idx = srcu_read_lock(&pmus_srcu);
> > > > -	list_for_each_entry_rcu(pmu, &pmus, entry) {
> > > > +	mutex_lock(&pmus_lock);
> > > 
> > > If the state change checked for by perf_pmu_register() needs to be also
> > > guarded by ctx->mutex, this looks right to me.
> 
> Right, so we have two locks, pmus_lock that serializes hotplug vs
> perf_pmu_register and ctx->lock that serializes find_get_context() vs
> hotplug.
> 
> Together they ensure that if a PMU is observed, the PMU's cpuctx's have
> the correct ->online state.

Sounds good, and now the added pmus_lock allows dropping get_online_cpus().

> > > Just for completeness, the other style is to maintain separate per-CPU
> > > state, in which case you would instead acquire pmus_lock, mark this
> > > CPU off-limits to more perf_pmu_register() usage, release pmus_lock,
> > > then clean up any old usage.
> 
> I'm not immediately seeing the other style, but per the above, I need
> that too. Because the previous could race against hotplug if
> perf_pmu_register() would observe cpu_online() as set after
> perf_event_exit_cpu() or something.
> 
> With the above change any chance of a race is gone and we don't need to
> worry about hotplug ordering too much.

Nice!!!

> Now I just need to do something about the swevent hash, because that's
> got a hole in now.

On the styles, here is a more coherence list:

1.	Use get_online_cpus(), but no CPU-hotplug notifiers.  You can
	use cpu_online() and cpu_is_offline() straightforwardly while
	get_online_cpus() is in effect, but these two may only be used
	for statistical/heuristic purposes otherwise.

2a.	Use CPU-hotplug notifiers, but not get_online_cpus().
	The notifiers maintain some sort of bitmask tracking which CPUs
	are present from the viewpoint of this subsystem.  This bitmask
	provides exact CPU presence/absence indications, at least
	assuming the appropriate lock is held.	The cpu_online() and
	cpu_is_offline() macros should be avoided, except possibly for
	statistical/heuristic purposes.

	For your perf patch, the bitmask is a cpumask_var_t.  For RCU,
	it is the combination of the ->qsmaskinitnext fields of the leaf
	rcu_node structures.

2b.	Like 2a, except that instead of a bitmask, the CPU online/offline
	information is implicit in other data structures.  For example,
	per-CPU structures might be allocated only when the corresponding
	CPU is online, so that a given per-CPU pointer is non-NULL
	iff the corresponding CPU is online.

So I was (confusingly) initially using "style" to distinguish #1 on the
one hand from #2a and #2b on the other.  Later on, I was using "style"
to distinguish #2a from #2b.

At this point, I am not sure that it makes all that much sense to
distinguish 2a from 2b.  Or you could argue that use of a cpumask_var_t
is its own substyle, with hand-crafted bitmasks (such as RCU's) are
separate substyles.  Can't say that I care all that much about that
fine-grained gradiations.  ;-)

							Thanx, Paul

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