Provide the necessary KCSAN checks to assist with debugging racy
use-after-frees.  While KASAN is more reliable at generally catching such
use-after-frees (due to its use of a quarantine), it can be difficult to
debug racy use-after-frees.  If a reliable reproducer exists, KCSAN can
assist in debugging such issues.

Note: ASSERT_EXCLUSIVE_ACCESS is a convenience wrapper if the size is
simply sizeof(var).  Instead, here we just use __kcsan_check_access()
explicitly to pass the correct size.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Link: http://lkml.kernel.org/r/20200623072653.114563-1-elver@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>