From: Uros Bizjak <ubizjak@gmail.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
loongarch@lists.linux.dev, linux-mips@vger.kernel.org,
linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org,
linux-perf-users@vger.kernel.org, Will Deacon <will@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
Boqun Feng <boqun.feng@gmail.com>
Subject: Re: [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks
Date: Tue, 4 Apr 2023 15:23:09 +0200 [thread overview]
Message-ID: <CAFULd4ZypxQULgq-MYgzsYd8k_BV0aH0eRhggX3MHWCgvKW=Bg@mail.gmail.com> (raw)
In-Reply-To: <ZCwj19okhYNRN8er@FVFF77S0Q05N.cambridge.arm.com>
On Tue, Apr 4, 2023 at 3:19 PM Mark Rutland <mark.rutland@arm.com> wrote:
>
> On Tue, Apr 04, 2023 at 02:24:38PM +0200, Uros Bizjak wrote:
> > On Mon, Apr 3, 2023 at 12:19 PM Mark Rutland <mark.rutland@arm.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@arm.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@arm.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.
>
> Thanks for this!
>
> FWIW, the patch (inline below) looks good to me.
Thanks, I will prepare a patch series for submission later today.
Uros.
next prev parent reply other threads:[~2023-04-04 13:23 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-05 20:56 [PATCH 00/10] locking: Introduce local{,64}_try_cmpxchg Uros Bizjak
2023-03-05 20:56 ` [PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks Uros Bizjak
2023-03-24 14:13 ` Mark Rutland
2023-03-24 15:43 ` Uros Bizjak
2023-03-24 16:14 ` Mark Rutland
2023-03-24 16:32 ` Mark Rutland
2023-03-26 19:28 ` Uros Bizjak
2023-04-03 10:19 ` Mark Rutland
2023-04-04 12:24 ` Uros Bizjak
2023-04-04 13:19 ` Mark Rutland
2023-04-04 13:23 ` Uros Bizjak [this message]
2023-03-05 20:56 ` [PATCH 02/10] locking/atomic: Add generic try_cmpxchg{,64}_local support Uros Bizjak
2023-03-05 20:56 ` [PATCH 03/10] locking/alpha: Wire up local_try_cmpxchg Uros Bizjak
2023-03-05 20:56 ` [PATCH 04/10] locking/loongarch: " Uros Bizjak
2023-03-05 20:56 ` [PATCH 05/10] locking/mips: " Uros Bizjak
2023-03-05 20:56 ` [PATCH 06/10] locking/powerpc: " Uros Bizjak
2023-03-05 20:56 ` [PATCH 07/10] locking/x86: " Uros Bizjak
2023-03-05 20:56 ` [PATCH 08/10] locking/generic: Wire up local{,64}_try_cmpxchg Uros Bizjak
2023-03-05 20:56 ` [PATCH 09/10] locking/x86: Enable local{,64}_try_cmpxchg Uros Bizjak
2023-03-05 20:56 ` [PATCH 10/10] perf/ring_buffer: use local_try_cmpxchg in __perf_output_begin Uros Bizjak
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