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

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-08-01 14:39:46 [+0200], Jason A. Donenfeld wrote:
> On RT, we can't call get_random_bytes() from inside of the raw locks
> that callers of vsprintf might take, because get_random_bytes() takes
> normal spinlocks. So on those RT systems, defer the siphash key
> generation to a worker.
> 
> Also, avoid using a static_branch, as this isn't the fast path.
> Using static_branch_likely() to signal that ptr_key has been filled is a
> bit much given that it is not a fast path.
> 
> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> Reported-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Sebastian - feel free to take this and tweak it as needed. Sending this
> mostly as something illustrative of what the "simpler" thing would be
> that I had in mind. -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)?

Sebastian