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

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Date:   Tue, 4 Apr 2023 14:24:38 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     Mark Rutland <mark.rutland@....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 Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@....com> wrote:
>
> On Sun, Mar 26, 2023 at 09:28:38PM +0200, Uros Bizjak wrote:
> > On Fri, Mar 24, 2023 at 5:33 PM Mark Rutland <mark.rutland@....com> wrote:
> > >
> > > 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 fallbacks are a bit more complicated than above, and are different
> > from atomic_try_cmpxchg.
> >
> > Please note in patch 2/10, the falbacks when arch_try_cmpxchg_local
> > are not defined call arch_cmpxchg_local. Also in patch 2/10,
> > try_cmpxchg_local is introduced, where it calls
> > arch_try_cmpxchg_local. Targets (and generic code) simply define (e.g.
> > :
> >
> > #define local_cmpxchg(l, o, n) \
> >        (cmpxchg_local(&((l)->a.counter), (o), (n)))
> > +#define local_try_cmpxchg(l, po, n) \
> > +       (try_cmpxchg_local(&((l)->a.counter), (po), (n)))
> >
> > which is part of the local_t API. Targets should either define all
> > these #defines, or none. There are no partial fallbacks as is the case
> > with atomic_t.
>
> Whether or not there are fallbacks is immaterial.
>
> In those cases, architectures can just as easily write C wrappers, e.g.
>
> long local_cmpxchg(local_t *l, long old, long new)
> {
>         return cmpxchg_local(&l->a.counter, old, new);
> }
>
> long local_try_cmpxchg(local_t *l, long *old, long new)
> {
>         return try_cmpxchg_local(&l->a.counter, old, new);
> }

Please find attached the complete prototype patch that implements the
above suggestion.

The patch includes:
- implementation of instrumented try_cmpxchg{,64}_local definitions
- corresponding arch_try_cmpxchg{,64}_local fallback definitions
- generic local{,64}_try_cmpxchg (and local{,64}_cmpxchg) C wrappers

- x86 specific local_try_cmpxchg (and local_cmpxchg) C wrappers
- x86 specific arch_try_cmpxchg_local definition

- kernel/events/ring_buffer.c change to test local_try_cmpxchg
implementation and illustrate the transition
- arch/x86/events/core.c change to test local64_try_cmpxchg
implementation and illustrate the transition

The definition of atomic_long_t is different for 64-bit and 32-bit
targets (s64 vs int), so target specific C wrappers have to use
different casts to account for this difference.

Uros.

View attachment "try_cmpxchg_local-04042023.diff.txt" of type "text/plain" (8903 bytes)