Crash when booting UML after e3c92e81711d14b46c3121d36bc8e152cb843923

[Linus Torvalds]

On Tue, 16 Jul 2024 at 17:53, Nathan Chancellor <nathan at kernel.org> wrote:
>
> Can confirm. I tried it initially and it did not seem to work but I
> think I may have been using the wrong object folder or something.

I wonder if our dependency rules might not be confused by a new header
file just showing up earlier in the search path.

So what might have happened for you is that it generated that new

    arch/um/include/generated/runtime-const.h

file thanks to the Kbuild rule, but then it didn't actually re-build
the fs/dcache.o file, because the way our auto-dependency works is
that it generates that list of all the other files it depends on, and
it also depends on the exact build flags - but that would all miss the
case of "there's a new file in the include path".

So that would explain your "it did not seem to work".

There's some handwaving here, but I do think that this is a case that
our otherwise pretty good automatic dependency rules may fail at.

              Linus