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@xxxxxxxxxx>
---
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
