[tip: locking/urgent] locking/lockdep: Use sched_clock() for random numbers

["tip-bot2 for Sebastian Andrzej Siewior" <tip-bot2@linutronix.de>]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     4051a81774d6d8e28192742c26999d6f29bc0e68
Gitweb:        https://git.kernel.org/tip/4051a81774d6d8e28192742c26999d6f29bc0e68
Author:        Sebastian Andrzej Siewior <bigeasy@linutronix.de>
AuthorDate:    Tue, 17 May 2022 11:16:14 +02:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 13 Jun 2022 10:29:57 +02:00

locking/lockdep: Use sched_clock() for random numbers

Since the rewrote of prandom_u32(), in the commit mentioned below, the
function uses sleeping locks which extracing random numbers and filling
the batch.
This breaks lockdep on PREEMPT_RT because lock_pin_lock() disables
interrupts while calling __lock_pin_lock(). This can't be moved earlier
because the main user of the function (rq_pin_lock()) invokes that
function after disabling interrupts in order to acquire the lock.

The cookie does not require random numbers as its goal is to provide a
random value in order to notice unexpected "unlock + lock" sites.

Use sched_clock() to provide random numbers.

Fixes: a0103f4d86f88 ("random32: use real rng for non-deterministic randomness")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/YoNn3pTkm5+QzE5k@linutronix.de
---
 kernel/locking/lockdep.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index 81e8728..f06b91c 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -5432,7 +5432,7 @@ static struct pin_cookie __lock_pin_lock(struct lockdep_map *lock)
 			 * be guessable and still allows some pin nesting in
 			 * our u32 pin_count.
 			 */
-			cookie.val = 1 + (prandom_u32() >> 16);
+			cookie.val = 1 + (sched_clock() & 0xffff);
 			hlock->pin_count += cookie.val;
 			return cookie;
 		}