[PATCH 01/10] locking/atomic: Add missing cast to try_cmpxchg() fallbacks

[Uros Bizjak]

On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland <mark.rutland at 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'.

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.