Re: [PATCH 2/6] test_hash.c: move common definitions to top of file

[Isabella B do Amaral]

Hello, Marco,

On Thu, Aug 26, 2021 at 11:36 AM Marco Elver <elver@xxxxxxxxxx> wrote:
>
> On Thu, 26 Aug 2021 at 03:26, 'Isabella Basso' via KUnit Development
> <kunit-dev@xxxxxxxxxxxxxxxx> wrote:
> > Keep function signatures minimal by making common definitions static.
> > This does not change any behavior.
>
> This seems like an odd change; if I read it right it's changing the
> out-param passed to test_int_hash() to simply be static globals.
>
> For one, it makes the code harder to read because now test_int_hash()
> is no longer "pure" (no global side-effects ... modulo printfs), and
> what was previously an out-param, is now a global.
>
> Unfortunately this is poor style and likely to lead to hard-to-debug
> problems. One such problem is if suddenly you have multiple threads
> involved. While this is just a test and unlikely to be a problem, I
> would recommend not introducing global state carelessly.

I see. My peers at LKCamp and I talked over the thread-safety problems for a
while but we concluded it wasn't a big deal (precisely because this is a test).
Though being stylistically poor seems a huge heads up, so I'm really thankful
for your thorough explanation(, and review)! Noted! :)

> An alternative common idiom, where a set of variables are always
> passed around to other functions, is to introduce a struct and pass a
> pointer to it along.
>
> > Signed-off-by: Isabella Basso <isabellabdoamaral@xxxxxx>
> > ---
> >  lib/test_hash.c | 13 ++++++++-----
> >  1 file changed, 8 insertions(+), 5 deletions(-)
> >
> > diff --git a/lib/test_hash.c b/lib/test_hash.c
> > index d4b0cfdb0377..8bcc645a7294 100644
> > --- a/lib/test_hash.c
> > +++ b/lib/test_hash.c
> > @@ -23,6 +23,11 @@
> >  #include <linux/stringhash.h>
> >  #include <linux/printk.h>
> >
> > +#define SIZE 256 /* Run time is cubic in SIZE */
> > +
> > +static u32 string_or; /* stores or-ed string output */
> > +static u32 hash_or[2][33] = { { 0, } }; /* stores or-ed hash output */
>
> These now use up memory for as long as this module is loaded, vs.
> before where it would only use up stack space. (For a test that's not
> a problem, but in non-test code it might.)
>
> >  /* 32-bit XORSHIFT generator.  Seed must not be zero. */
> >  static u32 __init __attribute_const__
> >  xorshift(u32 seed)
> > @@ -66,7 +71,7 @@ fill_buf(char *buf, size_t len, u32 seed)
> >   * recompile and re-test the module without rebooting.
> >   */
> >  static bool __init
> > -test_int_hash(unsigned long long h64, u32 hash_or[2][33])
> > +test_int_hash(unsigned long long h64)
> >  {
> >         int k;
> >         u32 h0 = (u32)h64, h1, h2;
> > @@ -123,17 +128,15 @@ test_int_hash(unsigned long long h64, u32 hash_or[2][33])
> >         return true;
> >  }
> >
> > -#define SIZE 256       /* Run time is cubic in SIZE */
> > -
> >  static int __init
> >  test_hash_init(void)
> >  {
> >         char buf[SIZE+1];
> > -       u32 string_or = 0, hash_or[2][33] = { { 0, } };
> >         unsigned tests = 0;
> >         unsigned long long h64 = 0;
> >         int i, j;
> >
> > +       string_or = 0;
>
> That's another problem with changes like this; now the compiler has no
> chance to warn you in case the variable is not initialized correctly.
>
> Also, I don't see string_or used anywhere else. Why make it global?
> If a later change would require that, it should say so in the commit
> message. But my guess is you can avoid all that by bundling everything
> up in a struct.
>
> >         fill_buf(buf, SIZE, 1);
> >
> >         /* Test every possible non-empty substring in the buffer. */
> > @@ -161,7 +164,7 @@ test_hash_init(void)
> >
> >                         string_or |= h0;
> >                         h64 = h64 << 32 | h0;   /* For use with hash_64 */
> > -                       if (!test_int_hash(h64, hash_or))
> > +                       if (!test_int_hash(h64))
> >                                 return -EINVAL;
> >                         tests++;
> >                 } /* i */
> > --
> > 2.33.0

Thanks,
--
Isabella Basso