Re: kernel build failure when CONFIG_DEBUG_INFO_BTF and CONFIG_KCSAN are enabled

[Yonghong Song <yonghong.song@linux.dev>]

On 11/26/25 2:29 AM, Nilay Shroff wrote:
> Hi,
>
> I am encountering the following build failures when compiling the kernel source checked out
> from the for-6.19/block branch [1]:
>
>    KSYMS   .tmp_vmlinux2.kallsyms.S
>    AS      .tmp_vmlinux2.kallsyms.o
>    LD      vmlinux.unstripped
>    BTFIDS  vmlinux.unstripped
> WARN: multiple IDs found for 'task_struct': 110, 3046 - using 110
> WARN: multiple IDs found for 'module': 170, 3055 - using 170
> WARN: multiple IDs found for 'file': 697, 3130 - using 697
> WARN: multiple IDs found for 'vm_area_struct': 714, 3140 - using 714
> WARN: multiple IDs found for 'seq_file': 1060, 3167 - using 1060
> WARN: multiple IDs found for 'cgroup': 2355, 3304 - using 2355
> WARN: multiple IDs found for 'inode': 553, 3339 - using 553
> WARN: multiple IDs found for 'path': 586, 3369 - using 586
> WARN: multiple IDs found for 'bpf_prog': 2565, 3640 - using 2565
> WARN: multiple IDs found for 'bpf_map': 2657, 3837 - using 2657
> WARN: multiple IDs found for 'bpf_link': 2849, 3965 - using 2849
> [...]
> make[2]: *** [scripts/Makefile.vmlinux:72: vmlinux.unstripped] Error 255
> make[2]: *** Deleting file 'vmlinux.unstripped'
> make[1]: *** [/home/src/linux/Makefile:1242: vmlinux] Error 2
> make: *** [Makefile:248: __sub-make] Error 2
>
>
> The build failure appears after commit 42adb2d4ef24 (“fs: Add the __data_racy annotation
> to backing_dev_info.ra_pages”) and commit 935a20d1bebf (“block: Remove queue freezing
> from several sysfs store callbacks”). However, the root cause does not seem to be specific
> to these commits or to the block layer changes themselves. Instead, the issue is triggered
> by the introduction of the __data_racy annotation on several fields in struct request_queue
> and struct backing_dev_info.
>
> It seems likely that some compilation units are built with KCSAN disabled, in which case
> the preprocessor expands __data_racy to nothing. Other units have KCSAN enabled, where
> __data_racy expands to the volatile qualifier. This results in two different versions
> of both struct request_queue and struct backing_dev_info: one where fields such as
> rq_timeout or ra_pages are declared volatile, and another where they are not.
>
> During BTF generation, the resolver encounters these conflicting type definitions.
> Although the reported error does not explicitly reference struct request_queue or
> struct backing_dev_info, it likely surfaces through types such as task_struct or
> others that embed these structures deep within their type hierarchies.
>
> For reference, gcc and pahole versions are shown below. Also, attached kernel .config:
>
> # gcc --version
> gcc (GCC) 11.4.1 20231218 (Red Hat 11.4.1-3)
> Copyright (C) 2021 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
>
> # pahole --version
> v1.27
>
> [1] https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git/log/?h=for-6.19/block

I tried with clang with CONFIG_DEBUG_INFO_BTF and CONFIG_KCSAN enabled.
I am using the latest bpf-next.
   $ make LLVM=1 -j
   ...
   KSYMS   .tmp_vmlinux0.kallsyms.S
   AS      .tmp_vmlinux0.kallsyms.o
   LD      .tmp_vmlinux1
   BTF     .tmp_vmlinux1
      <==== hang here

^Cmake[2]: *** [/home/yhs/work/bpf-next/scripts/Makefile.vmlinux:72: vmlinux.unstripped] Interrupt
make[1]: *** [/home/yhs/work/bpf-next/Makefile:1277: vmlinux] Interrupt
make: *** [/home/yhs/work/bpf-next/Makefile:248: __sub-make] Interrupt

I am using latest clang22 and latest pahole master (v1.31).

I will debug further from clang side.


>
> Thanks,
> --Nilay