Re: [PATCH] perf build: fix in-tree build

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Jan 24, 2025 at 02:06:08PM +0100, Luca Ceresoli wrote:
> Building perf in-tree is broken after commit 890a1961c812 ("perf tools:
> Create source symlink in perf object dir") which added a 'source' symlink
> in the output dir pointing to the source dir.
> 
> With in-tree builds, the added 'SOURCE = ...' line is executed multiple
> times (I observed 2 during the build plus 2 during installation). This is a
> minor inefficiency, in theory not harmful because symlink creation is
> assumed to be idempotent. But it is not.
> 
> Considering with in-tree builds:
> 
>   srctree=/absolute/path/to/linux
>    OUTPUT=/absolute/path/to/linux/tools/perf
> 
> here's what happens:
> 
>  1. ln -sf $(srctree)/tools/perf $(OUTPUT)/source
>     -> creates /absolute/path/to/linux/tools/perf/source
>        link to /absolute/path/to/linux/tools/perf
>     => OK, that's what was intended
>  2. ln -sf $(srctree)/tools/perf $(OUTPUT)/source   # same command as 1
>     -> creates /absolute/path/to/linux/tools/perf/perf
>        link to /absolute/path/to/linux/tools/perf
>     => Not what was intended, not idempotent
>  3. Now the build _should_ create the 'perf' executable, but it fails
> 
> The reason is the tricky 'ln' command line. At the first invocation 'ln'
> uses the 1st form:
> 
>        ln [OPTION]... [-T] TARGET LINK_NAME
> 
> and creates a link to TARGET *called LINK_NAME*.
> 
> At the second invocation $(OUTPUT)/source exists, so 'ln' uses the 3rd
> form:
> 
>        ln [OPTION]... TARGET... DIRECTORY
> 
> and creates a link to TARGET *called TARGET* inside DIRECTORY.
> 
> Fix by adding --no-dereference to "treat LINK_NAME as a normal file if it
> is a symbolic link to a directory", as the manpage says.
> 
> Link: https://lore.kernel.org/all/20241125182506.38af9907@booty/
> Fixes: 890a1961c812 ("perf tools: Create source symlink in perf object dir")
> Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
> ---
>  tools/perf/Makefile.perf | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/Makefile.perf b/tools/perf/Makefile.perf
> index d74241a151313bd09101aabb5d765a5a0a6efc84..bbd799a0fd544db220f29d1e250a819a765d04f3 100644
> --- a/tools/perf/Makefile.perf
> +++ b/tools/perf/Makefile.perf
> @@ -164,7 +164,7 @@ ifneq ($(OUTPUT),)
>  VPATH += $(OUTPUT)
>  export VPATH
>  # create symlink to the original source
> -SOURCE := $(shell ln -sf $(srctree)/tools/perf $(OUTPUT)/source)
> +SOURCE := $(shell ln -sf --no-dereference $(srctree)/tools/perf $(OUTPUT)/source)

The kernel Makefile has:
	$(Q)ln -fsn $(srcroot) source

So for parity the --no-dereference could become `n`, but it doesn't
really matter.

>  endif
>  
>  ifeq ($(V),1)
> 
> ---
> base-commit: ffd294d346d185b70e28b1a28abe367bbfe53c04
> change-id: 20250124-perf-fix-intree-build-fbd97f560254
> 
> Best regards,
> -- 
> Luca Ceresoli <luca.ceresoli@bootlin.com>
> 

Reviewed-by: Charlie Jenkins <charlie@rivosinc.com>
Tested-by: Charlie Jenkins <charlie@rivosinc.com>