linux-kernel - Re: arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly requires more registers than available

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Date: Mon, 24 Jun 2024 17:42:32 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: kernel test robot <lkp@...el.com>, oe-kbuild-all@...ts.linux.dev, 
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>
Subject: Re: arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly
 requires more registers than available

On Mon, Jun 24, 2024 at 4:18 PM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> On Mon, 24 Jun 2024 at 09:59, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
> >
> > On Mon, 24 Jun 2024 at 03:36, Uros Bizjak <ubizjak@...il.com> wrote:
> > >
> > > A real fix, not only a workaround, is to rewrite asm arguments to
> > > something like (untested, but "should work"TM):
> >
> > Sadly, I already tried that, and it didn't help.
>
> Bah. I _had_ tried that, but I had tried it on __arch_cmpxchg64_emu(),
> not on the "try" version.
>
> And no, it hadn't helped.
>
> But doing it *right*, and also doing __arch_try_cmpxchg64_emu() *does*
> actually help.
>
> Well, at least the attached patch compiles with the bad config and clang.
>
> I'm not certain it does the right thing, because I did more than just
> the %esi games to try to simplify it.
>
> It's *ENTIRELY* possible that I screwed up. Just in this thread, I
> have looked at the wrong inline asm at least twice now, so I'm not
> feeling super-competent right now.
>
> I'm sending the patch out in the hope that another set of eyes will
> make it actually better.

+ _lock "cmpxchg8b 0(%[ptr])", X86_FEATURE_CX8) \

This can be just:

+ _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \

- if (unlikely(!ret)) \
- *(_oldp) = o.full; \
+ *(_oldp) = o; \

This one should really update only when cmpxchg fails. The relation
between cmpxchg and try_cmpxchg is documented in
Documentation/atomic_t.txt, section CMPXCHG vs TRY_CMPXCHG.

Does the compilation still crash with the condition?

Uros.