On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote:
> 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@xxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> ---
> 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
>
