linux-kernel - Re: [PATCH 1/3] Make /dev/urandom scalable

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Message-ID: <20150924201413.GA3989@thunk.org>
Date:	Thu, 24 Sep 2015 16:14:13 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Austin S Hemmelgarn <ahferroin7@...il.com>
Cc:	Jeff Epler <jepler@...ythonic.net>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	kirill.shutemov@...ux.intel.com, herbert@...dor.apana.org.au,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 1/3] Make /dev/urandom scalable

On Thu, Sep 24, 2015 at 03:11:23PM -0400, Austin S Hemmelgarn wrote:
> >That is a startling result.  Please say what architecture, kernel
> >version, dieharder version and commandline arguments you are using to
> >get 10% WEAK or FAILED assessments from dieharder on /dev/urandom.
>
> I do not remember what exact dieharder version or command-line arguments
> (this was almost a decade ago), except that I compiled it from source
> myself, I do remember it was a 32-bit x86 processor (as that was sadly all I
> had to run Linux on at the time), and an early 2.6 series kernel (which if I
> remember correctly was already EOL by the time I was using it).

It might have been nice if you had said this from the beginning
instead of making an unqualified statement with the assumption that it
was applicable to kernels likely to be used today in non-obsolete
systems.  Otherwise it risks generating a click-bait article on
Phoronix that would get people really worried for no good reason...

There was a bug a long, long time ago (which where we weren't doing
sufficient locking and if two processes raced reading from
/dev/urandom at the same time, it was possible that the two processes
would get the same value read out from /dev/urandom).  This was fixed
a long time ago, though, and in fact the scalability problem which
Andi is trying to fix was caused by that extra locking that was added.  :-)

It's possible that is what you saw.  I don't know, since there was no
reproduction information to back up your rather startling claim.

If you can reproduce consistent Dieharder failures, please do let us
know with detailed reproduction instructures.

Many thanks,

					- Ted
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