Re: [PATCH v3 1/2] slab: support for compiler-assisted type-based slab cache partitioning

[Nathan Chancellor <nathan@kernel.org>]

On Wed, May 06, 2026 at 03:03:27PM +0200, Marco Elver wrote:
> Bah, this is why it doesn't work:
> 
> >> drivers/gpu/drm/msm/msm_gpu.c:272:4: error: cannot jump from this indirect goto statement to one of its possible targets
>      272 |                         drm_exec_retry_on_contention(&exec);
>          |                         ^
>    include/drm/drm_exec.h:123:4: note: expanded from macro
> 'drm_exec_retry_on_contention'
>      123 |                         goto *__drm_exec_retry_ptr;             \
>          |                         ^
>    drivers/gpu/drm/msm/msm_gpu.c:304:16: note: possible target of
> indirect goto statement
>      304 |                 state->bos = kcalloc(submit->nr_bos,
>          |                              ^
>    include/linux/slab.h:1173:34: note: expanded from macro 'kcalloc'
>     1173 | #define kcalloc(n, size, flags)         kmalloc_array(n,
> size, (flags) | __GFP_ZERO)
>          |                                         ^
>    include/linux/slab.h:1133:42: note: expanded from macro 'kmalloc_array'
>     1133 | #define kmalloc_array(...)
> alloc_hooks(kmalloc_array_noprof(__VA_ARGS__))
>          |                                                             ^
>    include/linux/slab.h:1132:71: note: expanded from macro
> 'kmalloc_array_noprof'
>     1132 | #define kmalloc_array_noprof(...)
> _kmalloc_array_noprof(__VA_ARGS__, __kmalloc_token(__VA_ARGS__))
>          |
>                        ^
>    include/linux/slab.h:506:55: note: expanded from macro '__kmalloc_token'
>      506 | #define __kmalloc_token(...) ((kmalloc_token_t){ .v = _THIS_IP_ })
>          |                                                       ^
>    include/linux/instruction_pointer.h:10:41: note: expanded from
> macro '_THIS_IP_'
>       10 | #define _THIS_IP_  ({ __label__ __here; __here: (unsigned
> long)&&__here; })
>          |                                         ^
>    drivers/gpu/drm/msm/msm_gpu.c:304:16: note: jump enters a statement
> expression
> 
> 
> Apparently using _THIS_IP_ creates a possible indirect jump target,
> but because it's in a statement expression, it's invalid, so the
> compiler complains. This is obviously nonsense, because the actual
> indirect jump in this gpu driver code would never jump to the
> _THIS_IP_ __here label, but that's what it is.
> 
> Given this pre-existing issue, we probably need to continue using
> _RET_IP_, as before. I tried to fix _THIS_IP_, but it's incredibly
> brittle (e.g. __always_inline function returning address of label
> doesn't work on Clang, but would on GCC).

For what it's worth, both LLVM and GCC consider the generic version of
_THIS_IP_ to be broken (even if it works currently), as the address of a
label is only expected to be used with a computed goto (hence the clang
error above, it just looks for possible computed goto targets):

  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=44298
  https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120071
  https://github.com/llvm/llvm-project/issues/138272

-- 
Cheers,
Nathan