linux-ext4 - Re: [GIT PULL] ext2, quota, and udf fixes for 6.6-rc1

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Date:   Fri, 20 Oct 2023 23:36:36 +0300
From:   andy.shevchenko@...il.com
To:     Jan Kara <jack@...e.cz>
Cc:     Andy Shevchenko <andriy.shevchenko@...el.com>,
        Baokun Li <libaokun1@...wei.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>, Jan Kara <jack@...e.cz>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Ferry Toth <ftoth@...londelft.nl>,
        linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org
Subject: Re: [GIT PULL] ext2, quota, and udf fixes for 6.6-rc1

Fri, Oct 20, 2023 at 12:43:56PM -0700, Linus Torvalds kirjoitti:
> On Fri, 20 Oct 2023 at 11:29, Andy Shevchenko
> <andriy.shevchenko@...el.com> wrote:
> >
> > I'll reply to this with the attached object file, I assume it won't go to the
> > mailing list, but should be available in your mailbox.
> 
> Honestly, both cases (that function gets inlined twice) look
> *identical* from a quick look, apart from obviously the extra call to
> __quota_error().
> 
> I might be missing something, but this most definitely is not a "gcc
> ends up creating very different code when it doesn't need to
> synchronize around the call" thing.
> 
> So a compiler issue looks very unlikely. No absolute guarantees - I
> didn't do *that* kind of walk-through instruction by instruction - but
> the results actually seem to line up perfectly.
> 
> Even register allocation didn't change, making the compare between #if
> 0 and without rather easy.
> 
> There's one extra spill/reload due to the call in the "non-#if0" case,
> and that actually made me look twice (because it spilled %eax, and
> then reloaded it as %rcx), but it turns that %eax/%ecx had the same
> value at the time of the spill, so even that was not a "real"
> difference.
> 
> So I will claim that no, it's not the compiler. It's some unrelated
> subtle timing, or possibly just a random code layout issue (because
> the code addresses do obviously change).

Okay, but since now I can't use the certain configuration, the bug is
persistent to me after this merge with the GCC. Yet, you mentioned that
you would expect some reports but I don't think many people have a
configuration similar to what I have. In any case a bug is lurking somewhere
there.

Let me check next week on different CPU (but I'm quite sceptical that it
may anyhow trigger the same behaviour as if it's a timing, many parameters
are involved, including hardware clocks, etc).

That said, if you or anyone has ideas how to debug futher, I'm all ears!

-- 
With Best Regards,
Andy Shevchenko