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@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Tested-by: Steven Rostedt <rostedt@goodmis.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: bpf@vger.kernel.org
Link: https://patch.msgid.link/8232efe8-a7a3-446c-af0b-19f9b523b4f7@paulmck-laptop
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index ea3f128de06fbc846244e23e0b309dafab63f98b..c4a0a93e8da4bf29604ce69d0b2f30918fda5b62 100644 (file)
--- 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);