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From: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Date: Wed, 8 May 2024 06:05:39 +0900
Message-ID: <CAKFNMonSDOWt20nq6UngSO1_7L-DLRMiAbcKXmquF04qbfC55Q@mail.gmail.com>
Subject: Re: [PATCH -mm] nilfs2: Use __field_struct() for a bitwise field
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>, Bart Van Assche <bvanassche@acm.org>, 
	linux-nilfs@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Rasmus Villemoes <linux@rasmusvillemoes.dk>
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On Wed, May 8, 2024 at 1:25=E2=80=AFAM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> On Tue, 7 May 2024 at 07:25, Ryusuke Konishi <konishi.ryusuke@gmail.com> =
wrote:
> >
> >    Despite that change, sparse complains when
> > passing a bitwise type to is_signed_type(). It is not clear to me why.
>
> Bah. The reason is this:
>
>    #define is_signed_type(type) (((type)(-1)) < (__force type)1)
>
> Basically, the way "is_signed_type()" works is that it casts a
> negative integer to the type, and checks to see if the value has now
> become a large value.
>
> Now, it looks odd, because only one of those casts has a "__force" on
> it, but the reason for that is that casting all-ones and all-zeroes is
> ok for bitwise types (think of bitwise types as being a "collection of
> bits" - so all bits set or all bits clear are sane concepts regardless
> of any other semantics).
>
> So it's not the casts themselves that are problematic: that part works fi=
ne.
>
> But you cannot compare a random collection of bits for greater than or
> lesser than.
>
> Think of things like byte orders: you can compare two values for
> _equality_ even if they are in the wrong byte order, but you can't
> compare them for "larger than" unless you turn them into the right CPU
> byte order.
>
> Basically, a "collection of bits" doesn't have an ordering in itself,
> even if equality comparisons are ok.
>
> So yeah, is_signed_type() doesn't work for bitwise types.
>
> And I don't see a sane way to make "is_signed_type()" to work for
> bitwise types - the whole concept of signedness of "bunch of bits" is
> kind of nonsensical - so I suspect your workaround is the best we can
> do (alternatively, tracing would have to figure out a different way to
> test for signedness).
>
>                  Linus

Linus, thank you very much for your detailed explanation.

I would like to edit the quoted part of his commit message

> >    Despite that change, sparse complains when
> > passing a bitwise type to is_signed_type(). It is not clear to me why.

as follows:

 Despite that change, sparse complains when passing a bitwise type
 to is_signed_type().  The reason is that in its definition below, a
 comparison will be made against bitwise types, which are
 random collections of bits (the casts to bitwise types themselves
 are semantically valid and are not problematic):

  #define is_signed_type(type) (((type)(-1)) < (__force type)1)

 So, as a workaround, fix the warnings by using __field_struct() macro
 that doesn't use is_signed_type() instead of __field().
 ...

I will try to resend the patch later unless there's a misunderstanding or
I'm missing too many points.

Thanks,
Ryusuke Konishi