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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hey again,

On Mon, Aug 01, 2022 at 03:36:32PM +0200, Jason A. Donenfeld wrote:
> Hi Sebastian,
> 
> On Mon, Aug 01, 2022 at 02:46:35PM +0200, Sebastian Andrzej Siewior wrote:
> > 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)?
> 
> "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?
> 
> 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

By the way, another option that would be fine with me would be to make
random.c use all raw spinlocks. From a non-RT perspective, that wouldn't
change the codegen at all, so it doesn't make a huge difference to me.
From an RT perspective, it would presumably fix a lot of these issues,
and enable randomness to be available in any context, which is maybe
what we want anyway. From an RT-safety point of view, I suspect doing
this might actually be okay, because the locks are only ever protecting
operations that are fixed duration CPU-bound, like generating a chacha
block or something, not waiting for some I/O.

Thoughts on that?

Jason