locks and kernel randomness...

[John-Mark Gurney]

Konstantin Belousov wrote this message on Tue, Feb 24, 2015 at 04:42 +0200:
> 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.

Well, the docs say:
     The speed and the maximum value of each counter is CPU-dependent.  Some
     CPUs (such as the Intel 80486) do not have such a register, so
     get_cyclecount() on these platforms returns a (monotonic) combination of
     numbers represented by the structure returned by binuptime(9).

The clang builtin of cycle counter is documented as returning 0 when
it isn't available.

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

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