linux-kernel - Re: [PATCH 01/10] locking/atomic: Add missing cast to try

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Date:   Fri, 24 Mar 2023 16:32:52 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Uros Bizjak <ubizjak@...il.com>
Cc:     linux-alpha@...r.kernel.org, linux-kernel@...r.kernel.org,
        loongarch@...ts.linux.dev, linux-mips@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-arch@...r.kernel.org,
        linux-perf-users@...r.kernel.org, Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg()
 fallbacks

On Fri, Mar 24, 2023 at 04:14:22PM +0000, Mark Rutland wrote:
> On Fri, Mar 24, 2023 at 04:43:32PM +0100, Uros Bizjak wrote:
> > On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland@....com> wrote:
> > >
> > > On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote:
> > > > Cast _oldp to the type of _ptr to avoid incompatible-pointer-types warning.
> > >
> > > Can you give an example of where we are passing an incompatible pointer?
> > 
> > An example is patch 10/10 from the series, which will fail without
> > this fix when fallback code is used. We have:
> > 
> > -       } while (local_cmpxchg(&rb->head, offset, head) != offset);
> > +       } while (!local_try_cmpxchg(&rb->head, &offset, head));
> > 
> > where rb->head is defined as:
> > 
> > typedef struct {
> >    atomic_long_t a;
> > } local_t;
> > 
> > while offset is defined as 'unsigned long'.
> 
> Ok, but that's because we're doing the wrong thing to start with.
> 
> Since local_t is defined in terms of atomic_long_t, we should define the
> generic local_try_cmpxchg() in terms of atomic_long_try_cmpxchg(). We'll still
> have a mismatch between 'long *' and 'unsigned long *', but then we can fix
> that in the callsite:
> 
> 	while (!local_try_cmpxchg(&rb->head, &(long *)offset, head))

Sorry, that should be:
	
	while (!local_try_cmpxchg(&rb->head, (long *)&offset, head))

The fundamenalthing I'm trying to say is that the
atomic/atomic64/atomic_long/local/local64 APIs should be type-safe, and for
their try_cmpxchg() implementations, the type signature should be:

	${atomictype}_try_cmpxchg(${atomictype} *ptr, ${inttype} *old, ${inttype} new)

Thanks,
Mark.