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:

quoted

On Fri, Mar 24, 2023 at 3:13 PM Mark Rutland [off-list ref] wrote:

quoted

On Sun, Mar 05, 2023 at 09:56:19PM +0100, Uros Bizjak wrote:

quoted

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.