A RT fix for lockdep. lockdep invokes prandom_u32() to create cookies. This

worked until prandom_u32() was switched to the real random generator, which
 takes a spinlock for extraction, which does not work on RT when invoked
 from atomic contexts. lockdep has no requirement for real random numbers
 and it turns out sched_clock() is good enough to create the cookie. That
 works everywhere and is faster.
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Merge tag 'locking-urgent-2022-06-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull lockdep fix from Thomas Gleixner:
 "A RT fix for lockdep.

  lockdep invokes prandom_u32() to create cookies. This worked until
  prandom_u32() was switched to the real random generator, which takes a
  spinlock for extraction, which does not work on RT when invoked from
  atomic contexts.

  lockdep has no requirement for real random numbers and it turns out
  sched_clock() is good enough to create the cookie. That works
  everywhere and is faster"

* tag 'locking-urgent-2022-06-19' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  locking/lockdep: Use sched_clock() for random numbers

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;
 		}