locks and kernel randomness...

[Warner Losh]

> On Feb 23, 2015, at 7:42 PM, Konstantin Belousov <kostikbel at gmail.com> wrote:
> 
> 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.

arm v4/v5 don’t have get_cyclecount() in hardware. It simply doesn’t exist.

However, this patch is only for SMP, which also isn’t available on arm v4/v5
in our tree.

MIPS’ get cycle count, though, has been defined since R4k days and so much
software depends on it, it would surprise me if that was eliminated to save silicon.

Then again, if you want to change random(), provide a weak_random() that’s
the traditional non-crypto thing that’s fast and lockless. That would make it easy
to audit in our tree. The scheduler doesn’t need cryptographic randomness, it
just needs to make different choices sometimes to ensure its notion of fairness.

Warner