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From: Valentin Schneider <valentin.schneider@arm.com>
To: linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	linux-rt-users@vger.kernel.org
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Ingo Molnar <mingo@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Joel Fernandes <joel@joelfernandes.org>,
	Anshuman Khandual <anshuman.khandual@arm.com>,
	Vincenzo Frascino <vincenzo.frascino@arm.com>,
	Steven Price <steven.price@arm.com>,
	Ard Biesheuvel <ardb@kernel.org>
Subject: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability
Date: Wed, 21 Jul 2021 12:51:17 +0100	[thread overview]
Message-ID: <20210721115118.729943-3-valentin.schneider@arm.com> (raw)
In-Reply-To: <20210721115118.729943-1-valentin.schneider@arm.com>

Running v5.13-rt1 on my arm64 Juno board triggers:

[    0.156302] =============================
[    0.160416] WARNING: suspicious RCU usage
[    0.164529] 5.13.0-rt1 #20 Not tainted
[    0.168300] -----------------------------
[    0.172409] kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state!
[    0.179920]
[    0.179920] other info that might help us debug this:
[    0.179920]
[    0.188037]
[    0.188037] rcu_scheduler_active = 1, debug_locks = 1
[    0.194677] 3 locks held by rcuc/0/11:
[    0.198448] #0: ffff00097ef10cf8 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip (./include/linux/rcupdate.h:662 kernel/softirq.c:171)
[    0.208709] #1: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: rt_spin_lock (kernel/locking/spinlock_rt.c:43 (discriminator 4))
[    0.217134] #2: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: __local_bh_disable_ip (kernel/softirq.c:169)
[    0.226428]
[    0.226428] stack backtrace:
[    0.230889] CPU: 0 PID: 11 Comm: rcuc/0 Not tainted 5.13.0-rt1 #20
[    0.237100] Hardware name: ARM Juno development board (r0) (DT)
[    0.243041] Call trace:
[    0.245497] dump_backtrace (arch/arm64/kernel/stacktrace.c:163)
[    0.249185] show_stack (arch/arm64/kernel/stacktrace.c:219)
[    0.252522] dump_stack (lib/dump_stack.c:122)
[    0.255947] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6439)
[    0.260328] rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58)
[    0.264537] rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777)
[    0.267786] rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876)
[    0.271644] smpboot_thread_fn (kernel/smpboot.c:165 (discriminator 3))
[    0.275767] kthread (kernel/kthread.c:321)
[    0.279013] ret_from_fork (arch/arm64/kernel/entry.S:1005)

In this case, this is the RCU core kthread accessing the local CPU's
rdp. Before that, rcu_cpu_kthread() invokes local_bh_disable().

Under !CONFIG_PREEMPT_RT (and rcutree.use_softirq=0), this ends up
incrementing the preempt_count, which satisfies the "local non-preemptible
read" of rcu_rdp_is_offloaded().

Under CONFIG_PREEMPT_RT however, this becomes

  local_lock(&softirq_ctrl.lock)

which, under the same config, is migrate_disable() + rt_spin_lock().
This *does* prevent the task from migrating away, but not in a way
rcu_rdp_is_offloaded() can notice. Note that the invoking task is an
smpboot thread, and thus cannot be migrated away in the first place.

Check is_pcpu_safe() here rather than preemptible().

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
---
 kernel/rcu/tree_plugin.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index ad0156b86937..6c3c4100da83 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp)
 		!(lockdep_is_held(&rcu_state.barrier_mutex) ||
 		  (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) ||
 		  rcu_lockdep_is_held_nocb(rdp) ||
-		  (rdp == this_cpu_ptr(&rcu_data) &&
-		   !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) ||
+		  (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) ||
 		  rcu_current_is_nocb_kthread(rdp) ||
 		  rcu_running_nocb_timer(rdp)),
 		"Unsafe read of RCU_NOCB offloaded state"
-- 
2.25.1


  parent reply	other threads:[~2021-07-21 11:58 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-21 11:51 [PATCH 0/3] sched: migrate_disable() vs per-CPU access safety checks Valentin Schneider
2021-07-21 11:51 ` [PATCH 1/3] sched: Introduce is_pcpu_safe() Valentin Schneider
2021-07-27 16:23   ` Paul E. McKenney
2021-07-21 11:51 ` Valentin Schneider [this message]
2021-07-27 16:24   ` [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Paul E. McKenney
2021-07-27 23:08   ` Frederic Weisbecker
2021-07-28 19:34     ` Valentin Schneider
2021-07-28 22:01       ` Frederic Weisbecker
2021-07-29  1:04         ` Paul E. McKenney
2021-07-29 10:51           ` Valentin Schneider
2021-07-21 11:51 ` [PATCH 3/3] arm64: mm: Make arch_faults_on_old_pte() check for migratability Valentin Schneider
2021-07-27 19:45 ` [PATCH 0/3] sched: migrate_disable() vs per-CPU access safety checks Thomas Gleixner

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