[PATCH 1/3] srcu: Warn when NMI-unsafe API is used in NMI

[Frederic Weisbecker <frederic@kernel.org>]

Using the NMI-unsafe reader API from within NMIs 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 ssp is in an NMI, RCU is definetly
   a better choice over SRCU.

3) Because of the previous reason (2), an ssp having an SRCU read side
   critical section in an NMI 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.

Signed-off-by: Frederic Weisbecker <frederic@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 c54142374793..8b7ef1031d89 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) {
-- 
2.25.1