Re: [PATCH v4 4/5] rcutorture: Force synchronizing of RCU flavor from hotplug notifier

["Paul E. McKenney" <paulmck@kernel.org>]

On Mon, Aug 10, 2020 at 01:31:09PM -0400, Joel Fernandes wrote:
> Hi Paul,
> 
> On Mon, Aug 10, 2020 at 09:19:45AM -0700, Paul E. McKenney wrote:
> > On Fri, Aug 07, 2020 at 01:07:21PM -0400, Joel Fernandes (Google) wrote:
> > > RCU has had deadlocks in the past related to synchronizing in a hotplug
> > > notifier. Typically, this has occurred because timer callbacks did not get
> > > migrated before the CPU hotplug notifier requesting RCU's services is
> > > called. If RCU's grace period processing has a timer callback queued in
> > > the meanwhile, it may never get called causing RCU stalls.
> > > 
> > > These issues have been fixed by removing such dependencies from grace
> > > period processing, however there are no testing scenarios for such
> > > cases.
> > > 
> > > This commit therefore reuses rcutorture's existing hotplug notifier to
> > > invoke the flavor-specific synchronize callback. If anything locks up,
> > > we expect stall warnings and/or other splats.
> > > 
> > > Obviously, we need not test for rcu_barrier from a notifier, since those
> > > are not allowed from notifiers. This fact is already detailed in the
> > > documentation as well.
> > > 
> > > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > 
> > Given that rcutorture_booster_init() is invoked on the CPU in question
> > only after it is up and running, and that (if I remember correctly)
> > rcutorture_booster_cleanup() is invoked on the outgoing CPU before it
> > has really started going away, would this code really have caught that
> > timer/CPU-hotplug/RCU bug?
> 
> You are right, it would not have caught that particular one because the timer
> callbacks would have been migrated by the time the rcutorture_booster_init()
> is called.
> 
> I still thought it is a good idea anyway to test if the dynamic hotplug
> notifiers don't have these issues.
> 
> Did you have a better idea on how to test the timer/hotplug/rcu bug?

My suggestion would be to place an rcutorture hook in all of the RCU
notifiers that support blocking and that have some possibility of making
this deadlock happen.  There are some similar hooks in other parts of RCU.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> 
> > >  kernel/rcu/rcutorture.c | 81 +++++++++++++++++++++--------------------
> > >  1 file changed, 42 insertions(+), 39 deletions(-)
> > > 
> > > diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
> > > index 92cb79620939..083b65e4877d 100644
> > > --- a/kernel/rcu/rcutorture.c
> > > +++ b/kernel/rcu/rcutorture.c
> > > @@ -1645,12 +1645,37 @@ rcu_torture_print_module_parms(struct rcu_torture_ops *cur_ops, const char *tag)
> > >  		 read_exit_delay, read_exit_burst);
> > >  }
> > >  
> > > -static int rcutorture_booster_cleanup(unsigned int cpu)
> > > +static bool rcu_torture_can_boost(void)
> > > +{
> > > +	static int boost_warn_once;
> > > +	int prio;
> > > +
> > > +	if (!(test_boost == 1 && cur_ops->can_boost) && test_boost != 2)
> > > +		return false;
> > > +
> > > +	prio = rcu_get_gp_kthreads_prio();
> > > +	if (!prio)
> > > +		return false;
> > > +
> > > +	if (prio < 2) {
> > > +		if (boost_warn_once  == 1)
> > > +			return false;
> > > +
> > > +		pr_alert("%s: WARN: RCU kthread priority too low to test boosting.  Skipping RCU boost test. Try passing rcutree.kthread_prio > 1 on the kernel command line.\n", KBUILD_MODNAME);
> > > +		boost_warn_once = 1;
> > > +		return false;
> > > +	}
> > > +
> > > +	return true;
> > > +}
> > > +
> > > +static int rcutorture_hp_cleanup(unsigned int cpu)
> > >  {
> > >  	struct task_struct *t;
> > >  
> > > -	if (boost_tasks[cpu] == NULL)
> > > +	if (!rcu_torture_can_boost() || boost_tasks[cpu] == NULL)
> > >  		return 0;
> > > +
> > >  	mutex_lock(&boost_mutex);
> > >  	t = boost_tasks[cpu];
> > >  	boost_tasks[cpu] = NULL;
> > > @@ -1662,11 +1687,14 @@ static int rcutorture_booster_cleanup(unsigned int cpu)
> > >  	return 0;
> > >  }
> > >  
> > > -static int rcutorture_booster_init(unsigned int cpu)
> > > +static int rcutorture_hp_init(unsigned int cpu)
> > >  {
> > >  	int retval;
> > >  
> > > -	if (boost_tasks[cpu] != NULL)
> > > +	/* Force synchronizing from hotplug notifier to ensure it is safe. */
> > > +	cur_ops->sync();
> > > +
> > > +	if (!rcu_torture_can_boost() || boost_tasks[cpu] != NULL)
> > >  		return 0;  /* Already created, nothing more to do. */
> > >  
> > >  	/* Don't allow time recalculation while creating a new task. */
> > > @@ -2336,30 +2364,6 @@ static void rcu_torture_barrier_cleanup(void)
> > >  	}
> > >  }
> > >  
> > > -static bool rcu_torture_can_boost(void)
> > > -{
> > > -	static int boost_warn_once;
> > > -	int prio;
> > > -
> > > -	if (!(test_boost == 1 && cur_ops->can_boost) && test_boost != 2)
> > > -		return false;
> > > -
> > > -	prio = rcu_get_gp_kthreads_prio();
> > > -	if (!prio)
> > > -		return false;
> > > -
> > > -	if (prio < 2) {
> > > -		if (boost_warn_once  == 1)
> > > -			return false;
> > > -
> > > -		pr_alert("%s: WARN: RCU kthread priority too low to test boosting.  Skipping RCU boost test. Try passing rcutree.kthread_prio > 1 on the kernel command line.\n", KBUILD_MODNAME);
> > > -		boost_warn_once = 1;
> > > -		return false;
> > > -	}
> > > -
> > > -	return true;
> > > -}
> > > -
> > >  static bool read_exit_child_stop;
> > >  static bool read_exit_child_stopped;
> > >  static wait_queue_head_t read_exit_wq;
> > > @@ -2503,8 +2507,7 @@ rcu_torture_cleanup(void)
> > >  		 rcutorture_seq_diff(gp_seq, start_gp_seq));
> > >  	torture_stop_kthread(rcu_torture_stats, stats_task);
> > >  	torture_stop_kthread(rcu_torture_fqs, fqs_task);
> > > -	if (rcu_torture_can_boost())
> > > -		cpuhp_remove_state(rcutor_hp);
> > > +	cpuhp_remove_state(rcutor_hp);
> > >  
> > >  	/*
> > >  	 * Wait for all RCU callbacks to fire, then do torture-type-specific
> > > @@ -2773,21 +2776,21 @@ rcu_torture_init(void)
> > >  		if (firsterr)
> > >  			goto unwind;
> > >  	}
> > > +
> > > +	firsterr = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "RCU_TORTURE",
> > > +			rcutorture_hp_init,
> > > +			rcutorture_hp_cleanup);
> > > +	if (firsterr < 0)
> > > +		goto unwind;
> > > +	rcutor_hp = firsterr;
> > > +
> > >  	if (test_boost_interval < 1)
> > >  		test_boost_interval = 1;
> > >  	if (test_boost_duration < 2)
> > >  		test_boost_duration = 2;
> > > -	if (rcu_torture_can_boost()) {
> > > -
> > > +	if (rcu_torture_can_boost())
> > >  		boost_starttime = jiffies + test_boost_interval * HZ;
> > >  
> > > -		firsterr = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "RCU_TORTURE",
> > > -					     rcutorture_booster_init,
> > > -					     rcutorture_booster_cleanup);
> > > -		if (firsterr < 0)
> > > -			goto unwind;
> > > -		rcutor_hp = firsterr;
> > > -	}
> > >  	shutdown_jiffies = jiffies + shutdown_secs * HZ;
> > >  	firsterr = torture_shutdown_init(shutdown_secs, rcu_torture_cleanup);
> > >  	if (firsterr)
> > > -- 
> > > 2.28.0.236.gb10cc79966-goog
> > >