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From:   Eric Dumazet <edumazet@google.com>
Date:   Thu, 31 Mar 2022 14:45:25 -0700
Message-ID: <CANn89iKaNEwyNZ=L_PQnkH0LP_XjLYrr_dpyRKNNoDJaWKdrmg@mail.gmail.com>
Subject: [BUG] rcu-tasks : should take care of sparse cpu masks
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>
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Hi Paul

It seems you assume per cpu ptr for arbitrary indexes (< nr_cpu_ids) are valid.

What do you think of the (untested) following patch ?

Thanks.

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 99cf3a13954cfb17828fbbeeb884f11614a526a9..df3785be4022e903d9682dd403464aa9927aa5c2
100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -273,13 +273,17 @@ static void call_rcu_tasks_generic(struct
rcu_head *rhp, rcu_callback_t func,
        bool needadjust = false;
        bool needwake;
        struct rcu_tasks_percpu *rtpcp;
+       int ideal_cpu, chosen_cpu;

        rhp->next = NULL;
        rhp->func = func;
        local_irq_save(flags);
        rcu_read_lock();
-       rtpcp = per_cpu_ptr(rtp->rtpcpu,
-                           smp_processor_id() >>
READ_ONCE(rtp->percpu_enqueue_shift));
+
+       ideal_cpu = smp_processor_id() >> READ_ONCE(rtp->percpu_enqueue_shift);
+       chosen_cpu = cpumask_next(ideal_cpu - 1, cpu_online_mask);
+
+       rtpcp = per_cpu_ptr(rtp->rtpcpu, chosen_cpu);
        if (!raw_spin_trylock_rcu_node(rtpcp)) { // irqs already disabled.
                raw_spin_lock_rcu_node(rtpcp); // irqs already disabled.
                j = jiffies;