[PATCH v3] rcu: Change return type of rcu_spawn_one_boost_kthread()

[Byungchul Park]

Hello,

I tested again if the WARN_ON_ONCE() is fired with my box.

And it was OK.

Thanks,
Byungchul

Changes from v2
-. Port the patch to a1af11a24cb0 (Paul's request)
-. Add a few new lines for a better look

Changes from v1
-. WARN_ON_ONCE() on failing to create rcu_boost_kthread.
-. Changed title and commit message a bit.

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