linux-kernel - Re: [PATCH -mm] nilfs2: Use __field

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Message-ID: <CAHk-=wgogPoSdCYw9jhc2Zm=BaE19nXYwFn_F9SwD2C-DyrmCw@mail.gmail.com>
Date: Tue, 7 May 2024 09:25:08 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Ryusuke Konishi <konishi.ryusuke@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-nilfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org, Bart Van Assche <bvanassche@....org>, 
	Rasmus Villemoes <linux@...musvillemoes.dk>
Subject: Re: [PATCH -mm] nilfs2: Use __field_struct() for a bitwise field

On Tue, 7 May 2024 at 07:25, Ryusuke Konishi <konishi.ryusuke@...il.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 fine.

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