linux-kernel - Re: [PATCH v2 1/3] base: mark 'no

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Date:   Mon, 26 Jul 2021 23:15:52 -0700
From:   Bill Wendling <morbo@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/3] base: mark 'no_warn' as unused

On Mon, Jul 26, 2021 at 10:27 PM Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Mon, Jul 26, 2021 at 01:47:33PM -0700, Nathan Chancellor wrote:
> > + Greg and Rafael as the maintainer and reviewer of drivers/base/module.c
> > respectively, drop everyone else.
>
> Odd no one cc:ed us originally, I guess they didn't want the patch ever
> merged?  :(
>
> >
> > Original post:
> >
> > https://lore.kernel.org/r/20210726201924.3202278-2-morbo@google.com/
> >
> > On 7/26/2021 1:19 PM, 'Bill Wendling' via Clang Built Linux wrote:
> > > Fix the following build warning:
> > >
> > >    drivers/base/module.c:36:6: error: variable 'no_warn' set but not used [-Werror,-Wunused-but-set-variable]
> > >          int no_warn;
>
> That's not going to be a good warning to ever have the kernel use due to
> how lots of hardware works (i.e. we need to do a read after a write but
> we can throw the read away as it does not matter).
>
>
> > >
> > > This variable is used to remove another warning, but causes a warning
> > > itself. Mark it as 'unused' to avoid that.
> > >
> > > Signed-off-by: Bill Wendling <morbo@...gle.com>
> >
> > Even though they evaluate to the same thing, it might be worth using
> > "__always_unused" here because it is :)
>
> But it is not unused, the value is written into it.
>
I believe that only matters if the variable is marked "volatile".
Otherwise, the variable itself is never used. A "variable that's
written to but not read from," in fact, is the whole reason for the
warning.

> So this isn't ok, sometimes we want to write to variables but never care
> about the value, that does not mean the compiler should complain about
> it.
>
Typically, if you don't care about the return value, you simply don't
assign it to a variable (cf. printf). However, the functions that
assign to "no_warn" have the "warn_unused_result" attribute. The fact
that the variable is named "no_warn" seems to indicate that it's meant
to remain unused, even if it probably should be checked.

Would you rather the warning be turned off on some level?

-bw