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From: Rasmus Villemoes <>
To: Andy Shevchenko <>
Cc: Shuah Khan <>, Kees Cook <>,
	Petr Mladek <>, Willy Tarreau <>,,,
	Arpitha Raghunandan <>,
	Brendan Higgins <>
Subject: Re: [PATCH 0/4] deterministic random testing
Date: Fri, 30 Oct 2020 13:58:42 +0100
Message-ID: <> (raw)
In-Reply-To: <>

On 26/10/2020 11.59, Andy Shevchenko wrote:
> On Sun, Oct 25, 2020 at 10:48:38PM +0100, Rasmus Villemoes wrote:
>> This is a bit of a mixed bag.
>> The background is that I have some sort() and list_sort() rework
>> planned, but as part of that series I want to extend their their test
>> suites somewhat to make sure I don't goof up - and I want to use lots
>> of random list lengths with random contents to increase the chance of
>> somebody eventually hitting "hey, sort() is broken when the length is
>> 3 less than a power of 2 and only the last two elements are out of
>> order". But when such a case is hit, it's vitally important that the
>> developer can reproduce the exact same test case, which means using a
>> deterministic sequence of random numbers.
>> Since Petr noticed [1] the non-determinism in test_printf in
>> connection with Arpitha's work on rewriting it to kunit, this prompted
>> me to use test_printf as a first place to apply that principle, and
>> get the infrastructure in place that will avoid repeating the "module
>> parameter/seed the rnd_state/report the seed used" boilerplate in each
>> module.
>> Shuah, assuming the kselftest_module.h changes are ok, I think it's
>> most natural if you carry these patches, though I'd be happy with any
>> other route as well.
> Completely in favour of this.
> Reviewed-by: Andy Shevchenko <>


> One note though. AFAIU the global variables are always being used in the
> modules that include the corresponding header. Otherwise we might have an extra
> warning(s). I believe you have compiled with W=1 to exclude other cases.

Yes, I unconditionally define the two new variables. gcc doesn't warn
about them being unused, since they are referenced from inside a

  if (0) {}

block. And when those references are the only ones, gcc is smart enough
to elide the static variables completely, so they don't even take up
space in .data (or - you can verify by running nm on
test_printf.o and test_bitmap.o - the former has 'seed' and 'rnd_state'
symbols, the latter does not.

I did it that way to reduce the need for explicit preprocessor
conditionals inside C functions.


      reply index

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-25 21:48 Rasmus Villemoes
2020-10-25 21:48 ` [PATCH 1/4] prandom.h: add *_state variant of prandom_u32_max Rasmus Villemoes
2020-10-30 16:00   ` Petr Mladek
2020-10-25 21:48 ` [PATCH 2/4] kselftest_module.h: unconditionally expand the KSTM_MODULE_GLOBALS() macro Rasmus Villemoes
2020-10-30 16:02   ` Petr Mladek
2020-10-25 21:48 ` [PATCH 3/4] kselftest_module.h: add struct rnd_state and seed parameter Rasmus Villemoes
2020-10-30 16:23   ` Petr Mladek
2020-10-25 21:48 ` [PATCH 4/4] lib/test_printf.c: use deterministic sequence of random numbers Rasmus Villemoes
2020-10-30 16:26   ` Petr Mladek
2020-10-26 10:59 ` [PATCH 0/4] deterministic random testing Andy Shevchenko
2020-10-30 12:58   ` Rasmus Villemoes [this message]

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