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

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Date:   Tue, 27 Oct 2020 13:38:59 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        Arnd Bergmann <arnd@...nel.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

From: Peter Zijlstra
> Sent: 27 October 2020 10:33
> 
> On Tue, Oct 27, 2020 at 09:33:32AM +0100, Arnd Bergmann wrote:
> > 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.
> 
> What is an actual geniune bug that this warning helps find?
> 
> Note that the kernel relies on -fno-strict-overflow to get rid of the
> signed UB that is otherwise present in C.
> 
> If you add that __diag_ignore() it should go in atomic.h I suppose,
> because all of atomic hard relies on this, and then the question becomes
> how much code is left that doesn't include that header and consequently
> doesn't ignore that warning.
> 
> So, is it useful to begin with in finding actual problems? and given we
> have to annotate away a bucket-load, how much coverage will there remain
> if we squish the known false-positives?

Especially since adding casts just makes the code harder to
read and can easily hide real bugs.

FWIW you might want to try -Wwrite-strings.
That ought to be fixable by sprinkling 'const.

	David

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