rcu: Allow rcu_irq_enter_check_tick() from NMI

Eugenio managed to tickle #PF from NMI context which resulted in
hitting a WARN in RCU through irqentry_enter() ->
__rcu_irq_enter_check_tick().

However, this situation is perfectly sane and does not warrant an
WARN. The #PF will (necessarily) be atomic and not require messing
with the tick state, so early return is correct.  This commit
therefore removes the WARN.

Fixes: aaf2bc50df1f ("rcu: Abstract out rcu_irq_enter_check_tick() from rcu_nmi_enter()")
Reported-by: "Eugenio Pérez" <eupm90@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 06895ef85d6926ec746bffff70933478a5db4049..93e1808ac3fcf1bd7d3bc8d19a893782c6696b77 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -928,8 +928,8 @@ void __rcu_irq_enter_check_tick(void)
 {
        struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
 
-        // Enabling the tick is unsafe in NMI handlers.
-       if (WARN_ON_ONCE(in_nmi()))
+       // If we're here from NMI there's nothing to do.
+       if (in_nmi())
                return;
 
        RCU_LOCKDEP_WARN(rcu_dynticks_curr_cpu_in_eqs(),