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Date: Mon, 10 Aug 2020 09:19:45 -0700
From: "Paul E. McKenney" <paulmck@...nel.org>
To: "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc: linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
Jonathan Corbet <corbet@....net>,
Josh Triplett <josh@...htriplett.org>,
Lai Jiangshan <jiangshanlai@...il.com>,
linux-doc@...r.kernel.org,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
neeraju@...eaurora.org, peterz@...radead.org,
Randy Dunlap <rdunlap@...radead.org>, rcu@...r.kernel.org,
Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de,
vineethrp@...il.com
Subject: Re: [PATCH v4 4/5] rcutorture: Force synchronizing of RCU flavor
from hotplug notifier
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@...lfernandes.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?
Thanx, Paul
> ---
> 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
>
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