linux-kernel - Re: [PATCH] qspinlock: use signed temporaries for cmpxchg

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Message-ID: <CAK8P3a2vUK5scbtcRTE98ZvwxMF3xMBT61JODV__RHMj+D0G2A@mail.gmail.com>
Date:   Tue, 27 Oct 2020 09:33:32 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Waiman Long <longman@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Will Deacon <will@...nel.org>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nicholas Piggin <npiggin@...il.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        linux-arch <linux-arch@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] qspinlock: use signed temporaries for cmpxchg

On Tue, Oct 27, 2020 at 8:47 AM Peter Zijlstra <peterz@...radead.org> wrote:
> On Mon, Oct 26, 2020 at 02:03:06PM -0400, Waiman Long wrote:
> > On 10/26/20 12:57 PM, Arnd Bergmann wrote:
> > Yes, it shouldn't really matter if the value is defined as int or u32.
> > However, the only caveat that I see is queued_spin_lock_slowpath() is
> > expecting a u32 argument. Maybe you should cast it back to (u32) when
> > calling it.
>
> No, we're not going to confuse the code. That stuff is hard enough as it
> is. This warning is garbage and just needs to stay off.

Ok, so the question then becomes: should we drop -Wpointer-sign from
W=2 and move it to W=3, or instead disable it locally. I could add
__diag_ignore(GCC, 4, "-Wpointer-sign") in the couple of header files
that produce this kind of warning if there is a general feeling that it
still helps to have this for drivers.

In the current state, there are a handful of header files that cause 90%
of all the W=2 warnings, making it impractical to ever build a driver
with W=2 and get anything useful out of it. I find some of the warnings
in the set useful in finding actual bugs, but much less so if they are
drowned out by noise from known false-positives.

     Arnd