From: Mark Rutland <mark.rutland@arm.com>
To: Uros Bizjak <ubizjak@gmail.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: Fri, 24 Mar 2023 16:14:22 +0000 [thread overview]
Message-ID: <ZB3MR8lGbnea9ui6@FVFF77S0Q05N> (raw)
In-Reply-To: <CAFULd4ZCgxDYnyy--qdgKoAo_y7MbNSaQdbdBFefnFuMoM2OYw@mail.gmail.com>
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))
... which then won't silently mask issues elsewhere, and will be consistent
with all the other atomic APIs.
Thanks,
Mark.
>
> The assignment in existing try_cmpxchg template:
>
> typeof(*(_ptr)) *___op = (_oldp)
>
> will trigger an initialization from an incompatible pointer type error.
>
> Please note that x86 avoids this issue by a cast in its
> target-dependent definition:
>
> #define __raw_try_cmpxchg(_ptr, _pold, _new, size, lock) \
> ({ \
> bool success; \
> __typeof__(_ptr) _old = (__typeof__(_ptr))(_pold); \
> __typeof__(*(_ptr)) __old = *_old; \
> __typeof__(*(_ptr)) __new = (_new); \
>
> so, the warning/error will trigger only in the fallback code.
>
> > That sounds indicative of a bug in the caller, but maybe I'm missing some
> > reason this is necessary due to some indirection.
> >
> > > Fixes: 29f006fdefe6 ("asm-generic/atomic: Add try_cmpxchg() fallbacks")
> >
> > I'm not sure that this needs a fixes tag. Does anything go wrong today, or only
> > later in this series?
>
> The patch at [1] triggered a build error in posix_acl.c/__get.acl due
> to the same problem. The compilation for x86 target was OK, because
> x86 defines target-specific arch_try_cmpxchg, but the compilation
> broke for targets that revert to generic support. Please note that
> this specific problem was recently fixed in a different way [2], but
> the issue with the fallback remains.
>
> [1] https://lore.kernel.org/lkml/20220714173819.13312-1-ubizjak@gmail.com/
> [2] https://lore.kernel.org/lkml/20221201160103.76012-1-ubizjak@gmail.com/
>
> Uros.
next prev parent reply other threads:[~2023-03-24 16:14 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 [this message]
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
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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