linux-kernel - [GIT pull] locking/urgent for 5.19-rc3

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Date:   Sun, 19 Jun 2022 15:12:35 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org
Subject: [GIT pull] locking/urgent for 5.19-rc3

Linus,

please pull the latest locking/urgent branch from:

   git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git locking-urgent-2022-06-19

up to:  4051a81774d6: locking/lockdep: Use sched_clock() for random numbers


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.

Thanks,

	tglx

------------------>
Sebastian Andrzej Siewior (1):
      locking/lockdep: Use sched_clock() for random numbers


 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 81e87280513e..f06b91ca6482 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;
 		}