[tip: core/rcu] rcu: Allow rcu_irq_enter_check_tick() from NMI

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     6dbce04d8417ae706596366e16841d77c454ba52
Gitweb:        https://git.kernel.org/tip/6dbce04d8417ae706596366e16841d77c454ba52
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Mon, 16 Nov 2020 13:10:12 +01:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Thu, 19 Nov 2020 19:34:17 -08:00

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@xxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Reviewed-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/tree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 06895ef..93e1808 100644
--- 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(),