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

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Date:   Tue, 27 Oct 2020 08:47:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Arnd Bergmann <arnd@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        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@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] qspinlock: use signed temporaries for cmpxchg

On Mon, Oct 26, 2020 at 02:03:06PM -0400, Waiman Long wrote:
> On 10/26/20 12:57 PM, Arnd Bergmann wrote:
> > From: Arnd Bergmann <arnd@...db.de>
> > 
> > When building with W=2, the build log is flooded with
> > 
> > include/asm-generic/qrwlock.h:65:56: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> > include/asm-generic/qrwlock.h:92:53: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> > include/asm-generic/qspinlock.h:68:55: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> > include/asm-generic/qspinlock.h:82:52: warning: pointer targets in passing argument 2 of 'atomic_try_cmpxchg_acquire' differ in signedness [-Wpointer-sign]
> > 
> > The atomics are built on top of signed integers, but the caller
> > doesn't actually care. Just use signed types as well.

Code consistency cares. Fundamentally we're treating it as a u32 here,
using int just because of a confused compiler warning will confuse.

> > @@ -77,7 +77,7 @@ extern void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
> >    */
> >   static __always_inline void queued_spin_lock(struct qspinlock *lock)
> >   {
> > -	u32 val = 0;
> > +	int val = 0;
> >   	if (likely(atomic_try_cmpxchg_acquire(&lock->val, &val, _Q_LOCKED_VAL)))
> >   		return;

> 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.