linux-kernel - Re: Crash when booting UML after e3c92e81711d14b46c3121d36bc8e152cb843923

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Message-ID: <CAHk-=whxxhjocp7Q92g4vkN75w+VNCEvAS+qbC8aEpguxudMAA@mail.gmail.com>
Date: Tue, 16 Jul 2024 19:01:10 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Nathan Chancellor <nathan@...nel.org>
Cc: linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: Crash when booting UML after e3c92e81711d14b46c3121d36bc8e152cb843923

On Tue, 16 Jul 2024 at 17:53, Nathan Chancellor <nathan@...nel.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