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rcu: Allow nesting of rcu_idle_enter() and rcu_idle_exit()
Use of RCU in the idle loop is incorrect, quite a few instances of just that have made their way into mainline, primarily event tracing. The problem with RCU read-side critical sections on CPUs that RCU believes to be idle is that RCU is completely ignoring the CPU, along with any attempts and RCU read-side critical sections. The approaches of eliminating the offending uses and of pushing the definition of idle down beyond the offending uses have both proved impractical. The new approach is to encapsulate offending uses of RCU with rcu_idle_exit() and rcu_idle_enter(), but this requires nesting for code that is invoked both during idle and and during normal execution. Therefore, this commit modifies rcu_idle_enter() and rcu_idle_exit() to permit nesting. Signed-off-by: Paul E. McKenney <firstname.lastname@example.org> Signed-off-by: Paul E. McKenney <email@example.com> Reviewed-by: Josh Triplett <firstname.lastname@example.org> Acked-by: Deepthi Dharwar <email@example.com>
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