[PATCH v3 rcu 09/11] srcu: Warn when NMI-unsafe API is used in NMI

["Paul E. McKenney" <paulmck@kernel.org>]

From: Frederic Weisbecker <frederic@kernel.org>

Using the NMI-unsafe reader API from within an NMI handler is very likely
to be buggy for three reasons:

1) NMIs aren't strictly re-entrant (a pending nested NMI will execute at
   the end of the current one) so it should be fine to use a non-atomic
   increment here. However, breakpoints can still interrupt NMIs and if
   a breakpoint callback has a reader on that same ssp, a racy increment
   can happen.

2) If the only reader site for a given srcu_struct structure is in an
   NMI handler, then RCU should be used instead of SRCU.

3) Because of the previous reason (2), an srcu_struct structure having
   an SRCU read side critical section in an NMI handler is likely to
   have another one from a task context.

For all these reasons, warn if an NMI-unsafe reader API is used from an
NMI handler.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 kernel/rcu/srcutree.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 87ae6f5c1edae..18bb696cff8ca 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe)
 
 	if (!IS_ENABLED(CONFIG_PROVE_RCU))
 		return;
+	/* NMI-unsafe use in NMI is a bad sign */
+	WARN_ON_ONCE(!nmi_safe && in_nmi());
 	sdp = raw_cpu_ptr(ssp->sda);
 	old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety);
 	if (!old_nmi_safe_mask) {
-- 
2.31.1.189.g2e36527f23