I agree.

The memory operations upon the object should be adjacent, and there
should be no external call between them.  mtx_enter() is (potentially?)
an external call in the global scope.

If the memory operation are adjacent, a compiler can decide upon the
order of writes and avoid extras, because memset is known to the compiler.

> Index: kern/kern_sysctl.c
> ===================================================================
> RCS file: /data/mirror/openbsd/cvs/src/sys/kern/kern_sysctl.c,v
> diff -u -p -r1.467 kern_sysctl.c
> --- kern/kern_sysctl.c	6 May 2025 18:34:26 -0000	1.467
> +++ kern/kern_sysctl.c	8 May 2025 12:58:42 -0000
> @@ -598,8 +598,8 @@ kern_sysctl(int *name, u_int namelen, vo
>  		} else 
>  			return (0);
>  
> -		memset(&ump, 0, sizeof(ump));
>  		mtx_enter(&log_mtx);
> +		memset(&ump, 0, sizeof(ump));
>  		ump.msg_magic = mp->msg_magic;
>  		ump.msg_bufs = mp->msg_bufs;
>  		ump.msg_bufx = mp->msg_bufx;
>