linux-kernel - Re: [PATCH v4] lib/vsprintf: defer filling siphash key on RT

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Date:   Mon, 1 Aug 2022 15:47:03 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-kernel@...r.kernel.org,
        Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        John Ogness <john.ogness@...utronix.de>,
        Mike Galbraith <efault@....de>, Petr Mladek <pmladek@...e.com>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Theodore Ts'o <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4] lib/vsprintf: defer filling siphash key on RT

On 2022-08-01 15:36:32 [+0200], Jason A. Donenfeld wrote:
> Hi Sebastian,
Hi Jason,

> > Can have the same behaviour regardless of CONFIG_PREEMPT_RT? Here
> > lockdep _may_ yell with !RT because it is broken for RT.
> > If we agree that we drop the first %p print here, can we do this on
> > both (regardless of CONFIG_PREEMPT_RT)?
> 
> "Lockdep may yell" -- but this would be when lockdep is turned on to
> catch RT bugs, not to catch non-RT bugs. The actual bug only exists on
> RT. This is an RT problem. Stop pretending that this is a real issue
> outside of RT. It isn't. This is *only* an RT issue. So why would we
> make things worse for an issue that doesn't actually exist on non-RT?

You do remember the warning that poped up in random core with
CONFIG_PROVE_RAW_LOCK_NESTING enabled? Where I said it does not affect
!RT it just points out a RT problem in a !RT config?
If you fix this with one code path for RT and another one for !RT then
you will have this warning _if_ the caller has a raw_spinlock_t
acquired.

> I too generally prefer having only one code path and not two. But the
> way this patch is written, the worker function just gets reused with a
> straight call on the non-RT case, so it doesn't actually require
> duplicating code.

> Jason

Sebastian