locks and kernel randomness...

[Konstantin Belousov]

On Mon, Feb 23, 2015 at 06:04:12PM -0800, Harrison Grundy wrote:
> 
> 
> On 02/23/15 17:57, Konstantin Belousov wrote:
> > 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'.
> > 
> > Please do not enforce yet another spinlock for the system. 
> > _______________________________________________
> 
> The patch attached to
> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=197922 switches
> sched_balance to use get_cyclecount, which is also a suitable source
> of entropy for this purpose.
> 
> It would also be possible to make the scheduler deterministic here,
> using cpuid or some such thing to make sure all CPUs don't fire the
> balancer at the same time.
> 

The patch in the PR is probably in the right direction, but might be too
simple, unless somebody dispel my fallacy. I remember seeing claims that
on the very low-end embedded devices the get_cyclecount() method may
be non-functional, i.e. returning some constant, probably 0. I somehow
associate MIPS arch with this bias.