locks and kernel randomness...

[John-Mark Gurney]

Konstantin Belousov wrote this message on Tue, Feb 24, 2015 at 03:57 +0200:
> On Mon, Feb 23, 2015 at 05:20:26PM -0800, John-Mark Gurney wrote:
> > I'm working on simplifying kernel randomness interfaces.  I would like
> > to get read of all weak random generators, and this means replacing
> > read_random and random(9) w/ effectively arc4rand(9) (to be replaced
> > by ChaCha or Keccak in the future).
> > 
> > The issue is that random(9) is called from any number of contexts, such
> > as the scheduler.  This makes locking a bit more interesting.  Currently,
> > both arc4rand(9) and yarrow/fortuna use a default mtx lock to protect
> > their state.  This obviously isn't compatible w/ the scheduler, and
> > possibly other calling contexts.
> > 
> > I have a patch[1] that unifies the random interface.  It converts a few
> > of the locks from mtx default to mtx spin to deal w/ this.
> This is definitely an overkill. The rebalancing minor use of randomness
> absolutely does not require cryptographical-strenght randomness to
> select a moment to rebalance thread queue. Imposing the spin lock on
> the whole random machinery just to allow the same random gathering code
> to be used for balance_ticks is detriment to the system responsivness.
> Scheduler is fine even with congruential generators, as you could see in
> the cpu_search(), look for the '69069'.

Then why doesn't it use this then?  This is exactly why I don't want
random to be a congruential generator... If you're so sure that you
don't need cryptographic secure and that it's a performance benefit
to do so, then you're free to roll your own, but almost all of the
time, code won't meet both requirements.

I haven't audited all the places where random is currently being called
that might require not sleeping.  The scheduler happens to be the first
one I ran into...

> Please do not enforce yet another spinlock for the system.

I sent this email asking for help for how to avoid a spin lock.  I'd
appreciate if you could suggest how to improve my patch.

Thanks.

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."