[PATCH] srcu: Fix warning to permit SRCU-fast readers in NMI handlers

[Paul E. McKenney]

srcu: Fix warning to permit SRCU-fast readers in NMI handlers

SRCU-fast is designed to be used in NMI handlers, even going so far
as to use atomic operations for architectures supporting NMIs but not
providing NMI-safe per-CPU atomic operations.  However, the WARN_ON_ONCE()
in __srcu_check_read_flavor() complains if SRCU-fast is used in an NMI
handler.  This commit therefore modifies that WARN_ON_ONCE() to avoid
such complaints.

Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
Tested-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Andrii Nakryiko <andrii@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: bpf@xxxxxxxxxxxxxxx

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index c469c708fdd6a..66ba6a2f83d3a 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -789,7 +789,8 @@ void __srcu_check_read_flavor(struct srcu_struct *ssp, int read_flavor)
 	struct srcu_data *sdp;
 
 	/* NMI-unsafe use in NMI is a bad sign, as is multi-bit read_flavor values. */
-	WARN_ON_ONCE((read_flavor != SRCU_READ_FLAVOR_NMI) && in_nmi());
+	WARN_ON_ONCE(read_flavor != SRCU_READ_FLAVOR_NMI &&
+		     read_flavor != SRCU_READ_FLAVOR_FAST && in_nmi());
 	WARN_ON_ONCE(read_flavor & (read_flavor - 1));
 
 	sdp = raw_cpu_ptr(ssp->sda);