Re: [PATCH] locking/lockdep: Use sched_clock() for random numbers.

[Sasha Levin]

On Tue, May 17, 2022 at 11:16:14AM +0200, Sebastian Andrzej Siewior 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@xxxxxxxxxxxxx>
> ---
>
> 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.

I'm seeing hangs on AMD based machines with lockdep enabled, and
bisected the issue to a0103f4d86f88 ("random32: use real rng for
non-deterministic randomness").

No hangs on Intel/arm64, no hangs with lockdep disabled.

Spotted this patch on the list, and confirmed that it addresses the
issue I'm seeing.

--
Thanks,
Sasha