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Subject[BUG] rcu-tasks : should take care of sparse cpu masks
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;
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