[PATCH] locking/lockdep: Disable KASAN instrumentation of lockdep.c

[Waiman Long <longman@redhat.com>]

Both KASAN and LOCKDEP are commonly enabled in building a debug kernel.
Each of them can significantly slow down the speed of a debug kernel.
Enabling KASAN instrumentation of the LOCKDEP code will further slow
thing down.

Since LOCKDEP is a high overhead debugging tool, it will never get
enabled in a production kernel. The LOCKDEP code is also pretty mature
and is unlikely to get major changes. There is also a possibility of
recursion similar to KCSAN. As the small advantage of enabling KASAN
instrumentation to catch potential memory access error is probably
not worth the drawback of further slowing down a debug kernel, disable
KASAN instrumentation to enable a debug kernel to gain a little bit of
speed back.

With a debug kernel with both LOCKDEP and KASAN enabled running on a
2-socket 144-thread system, the time to do a "make -j144" kernel build
was 18m40.641s. After applying this patch, the parallel kernel build
time was reduced to 17m35.136s. This is a reduction of about 66s (5.8%).

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/Makefile | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 0db4093d17b8..8a588b0227b1 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -6,6 +6,7 @@ KCOV_INSTRUMENT		:= n
 obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o
 
 # Avoid recursion lockdep -> sanitizer -> ... -> lockdep.
+KASAN_SANITIZE_lockdep.o := n
 KCSAN_SANITIZE_lockdep.o := n
 
 ifdef CONFIG_FUNCTION_TRACER
-- 
2.48.1