linux-kernel - Re: [PATCH] locking/lockdep: Use sched

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Message-ID: <Yp+CCGIdrIFZbJKs@linutronix.de>
Date:   Tue, 7 Jun 2022 18:51:20 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     linux-kernel@...r.kernel.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Theodore Ts'o <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] locking/lockdep: Use sched_clock() for random numbers.

On 2022-05-17 11:16:14 [+0200], To linux-kernel@...r.kernel.org wrote:
> 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@...utronix.de>
> ---
> 
> So if the architecture does not provide sched_clock() and does not
> enabled GENERIC_SCHED_CLOCK then we use jiffies here. Most of them do
> one or the other except for alpha, csky, hexagon, ... but I don't worry
> here since arm*, power*, x86* do provide it.

ping.

Jason suggested atomic_inc_return(&some_global).

>  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 81e87280513ea..f06b91ca6482d 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;
>  		}

Sebastian