Re: [PATCH 51/51] fs/zonefs: Fix sparse warnings in tracing code

From: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Bart Van Assche <bvanassche@acm.org>,
	Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
	linux-block <linux-block@vger.kernel.org>,
	Damien Le Moal <damien.lemoal@opensource.wdc.com>,
	Naohiro Aota <naohiro.aota@wdc.com>,
	Johannes Thumshirn <jth@kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sparse Mailing-list <linux-sparse@vger.kernel.org>
Subject: Re: [PATCH 51/51] fs/zonefs: Fix sparse warnings in tracing code
Date: Sun, 26 Jun 2022 22:10:02 +0200	[thread overview]
Message-ID: <20220626201002.net2d4clpe23j24n@mail> (raw)
In-Reply-To: <CAHk-=whMV0CAb66pJASCcAwMZNfJpRKrO=EwXcm-vWUnXF7gDA@mail.gmail.com>

On Sun, Jun 26, 2022 at 09:50:50AM -0700, Linus Torvalds wrote:
> On Sun, Jun 26, 2022 at 9:33 AM Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
> >
> >  (a) for the casting part, I actually suspect we should drop the
> > warning about castign integers to restricted types.
> 
> Heh. Trying that out, I find we seem to do this at the wrong point anyway.
> 
> I removed the check for "value of 0 is ok", and it continues to warn
> about casting "-1".
> 
> Because it does that before "-1" has even been simplified, so it
> actually sees it as an *expression* ("negate the constant 1") rather
> than as the *value* -1, and it warns.
> 
> That's a bit sad since really -1 and ~0 really both should be ok for
> the same reason plain zero is ok - regardless of the whole "do we just
> allow it for constants in general" thing.

Well, this "there is no negative constants, only minus positive ones" 
is kinda special. Maybe we can special-case this, just doing an early
mini-expansion of PREOP('-', EXPR_VALUE). I'm not sure about all
implications it would have on typechecking though.

> I think the bitwise thing was always a bit half-baked. It was designed
> for detecting little-endian and big-endian issues, and "bitmask"
> types. And it's wonderful for the basics of that, but I think it was
> never really thought through for this kind of extended use where we
> end up having generic macros that do still make sense for them, but
> aren't that exact "avoid assigning bitwise values to other values"

Yes, I fully agree
Also, it's the only 'strong' type that sparse has (IIRC nocast was
too weak), so it's used for everything. Allowing only the bitwise
operators is often too restrictive. I think that often what people
just need is an unique type that doesn't mix with other types
but that can 1) mix with constants (certainly with 0, 1 and -1) and
2) can use the usual arithmetic operations (certainly the compares).

-- Luc