[bug report] percpu: Wire up cmpxchg128

[Dan Carpenter <dan.carpenter@linaro.org>]

Hello Peter Zijlstra,

This code is old, and so it must work.  I can't explain why the warning
is only showing up now.  I was just curious what's going on.

Commit 6d12c8d308e6 ("percpu: Wire up cmpxchg128") from May 31, 2023
(linux-next), leads to the following Smatch static checker warning:

	mm/slub.c:3609 __update_cpu_freelist_fast()
	warn: sizeof(void)

mm/slub.c
    3599 
    3600 #ifndef CONFIG_SLUB_TINY
    3601 static inline bool
    3602 __update_cpu_freelist_fast(struct kmem_cache *s,
    3603                            void *freelist_old, void *freelist_new,
    3604                            unsigned long tid)
    3605 {
    3606         freelist_aba_t old = { .freelist = freelist_old, .counter = tid };
    3607         freelist_aba_t new = { .freelist = freelist_new, .counter = next_tid(tid) };
    3608 
--> 3609         return this_cpu_try_cmpxchg_freelist(s->cpu_slab->freelist_tid.full,
    3610                                              &old.full, new.full);
    3611 }

The problem is:

arch/arm64/include/asm/percpu.h
   239  #define this_cpu_cmpxchg128(pcp, o, n)                                  \
   240  ({                                                                      \
   241          typedef typeof(pcp) pcp_op_T__;                                 \
   242          u128 old__, new__, ret__;                                       \
   243          pcp_op_T__ *ptr__;                                              \
   244          old__ = o;                                                      \
   245          new__ = n;                                                      \
   246          preempt_disable_notrace();                                      \
   247          ptr__ = raw_cpu_ptr(&(pcp));                                    \
   248          ret__ = cmpxchg128_local((void *)ptr__, old__, new__);          \
                                         ^^^^^^^^^^^^^^
We're passing a void pointer.

   249          preempt_enable_notrace();                                       \
   250          ret__;                                                          \
   251  })

include/linux/atomic/atomic-instrumented.h
  5034  #define try_cmpxchg128_local(ptr, oldp, ...) \
  5035  ({ \
  5036          typeof(ptr) __ai_ptr = (ptr); \
  5037          typeof(oldp) __ai_oldp = (oldp); \
  5038          instrument_atomic_read_write(__ai_ptr, sizeof(*__ai_ptr)); \
                                                       ^^^^^^^^^^^^^^^^^
Here we're doing a sizeof() of a void pointer which is 1.  I think maybe
this is just for KASAN so it's not a big deal in terms of affecting
runtime?

  5039          instrument_read_write(__ai_oldp, sizeof(*__ai_oldp)); \
  5040          raw_try_cmpxchg128_local(__ai_ptr, __ai_oldp, __VA_ARGS__); \
  5041  })


regards,
dan carpenter