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McKenney" Cc: linux-kernel@vger.kernel.org, Davidlohr Bueso , Jonathan Corbet , Josh Triplett , Lai Jiangshan , linux-doc@vger.kernel.org, Mathieu Desnoyers , Mauro Carvalho Chehab , neeraju@codeaurora.org, peterz@infradead.org, Randy Dunlap , rcu@vger.kernel.org, Steven Rostedt , tglx@linutronix.de, vineethrp@gmail.com Subject: Re: [PATCH v4 4/5] rcutorture: Force synchronizing of RCU flavor from hotplug notifier Message-ID: <20200810194131.GE2865655@google.com> References: <20200807170722.2897328-1-joel@joelfernandes.org> <20200807170722.2897328-5-joel@joelfernandes.org> <20200810161945.GK4295@paulmck-ThinkPad-P72> <20200810173109.GA2253395@google.com> <20200810175434.GL4295@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200810175434.GL4295@paulmck-ThinkPad-P72> Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Aug 10, 2020 at 10:54:34AM -0700, Paul E. McKenney wrote: > 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) > > > > > > 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. Sure that's a good idea, I will look into it. Thanks! - Joel > 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 > > > >