[PATCH] kernel/printk: prevent deadlock at calling kmsg_dump from NMI context

[Konstantin Khlebnikov <khlebnikov@yandex-team.ru>]

Function kmsg_dump could be invoked from NMI context intentionally or
accidentally because it is called at various oops/panic paths.
Kernel message dumpers are not ready to work in NMI context right now.
They could deadlock on lockbuf_lock or break internal structures.

Theoretically some dumpers could be fixed and marked as NMI-safe.
Right now it's safer to print warning once and give up.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
 kernel/printk/printk.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 1888f6a3b694..2b83820bdc57 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -3104,6 +3104,9 @@ void kmsg_dump(enum kmsg_dump_reason reason)
 	struct kmsg_dumper *dumper;
 	unsigned long flags;
 
+	if (WARN_ON_ONCE(in_nmi()))
+		return;
+
 	if ((reason > KMSG_DUMP_OOPS) && !always_kmsg_dump)
 		return;