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

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Message-ID: <CAFULd4YVOwxQ4JDaOdscX_vtJsqJBJ5zhd0RtXXutW=Eqh29Qw@mail.gmail.com>
Date: Sun, 23 Jun 2024 12:14:32 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: kernel test robot <lkp@...el.com>
Cc: 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>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly
 requires more registers than available

On Sun, Jun 23, 2024 at 4:02 AM kernel test robot <lkp@...el.com> wrote:
>
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> head:   5f583a3162ffd9f7999af76b8ab634ce2dac9f90
> commit: 95ece48165c136b96fae0f6144f55cbf8b24aeb9 locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}() functions
> date:   2 months ago
> config: i386-randconfig-061-20240623 (https://download.01.org/0day-ci/archive/20240623/202406230912.F6XFIyA6-lkp@intel.com/config)
> compiler: clang version 18.1.5 (https://github.com/llvm/llvm-project 617a15a9eac96088ae5e9134248d8236e34b91b1)
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20240623/202406230912.F6XFIyA6-lkp@intel.com/reproduce)
>
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@...el.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202406230912.F6XFIyA6-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
>    In file included from kernel/bpf/core.c:21:
>    In file included from include/linux/filter.h:8:
>    In file included from include/linux/atomic.h:7:
>    In file included from arch/x86/include/asm/atomic.h:8:
>    In file included from arch/x86/include/asm/cmpxchg.h:143:
> >> arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly requires more registers than available
>      149 |         return __arch_try_cmpxchg64_emu(ptr, oldp, new);

The referred patch actually mitigates the issue, as explained in
details here [1].

[1] https://lore.kernel.org/lkml/CAFULd4b8kRgy0p3pKwheLRHx7rX+4RuY-45_Zt-CCj5sCpGjCw@mail.gmail.com/

Based on the above explanation, the build will break even harder
without the patch, because *TWO* additional registers are needed to
carry the value to be compared over cmpxchg8b instruction. The trouble
that register allocator goes through can be seen at [2] and [3].

[2] https://lore.kernel.org/lkml/20240410062957.322614-1-ubizjak@gmail.com/
[3] https://lore.kernel.org/lkml/20240410062957.322614-3-ubizjak@gmail.com/

Up to now, the atomic64_{and,or,xor} functions (and their derivatives)
were/are seldom used in the kernel. Recently, their usage increased,
so the register pressure situation around cmpxchg8b instruction
worsened. For some reason, x86 implementations of these functions are
declared as inline functions, in contrast with much more used
atomic64_{inc,dec,add,sub}, which are declared as outline functions.
While using try_cmpxchg() instead of cmpxchg() helps, it can only go
so far.

The real and reliable fix is to re-declare x86
arch_atomic64_{and,or,xor} functions as outline functions in the same
way x86 arch_atomic64_{inc,dec,add,sub} are declared. The
infrastructure is already there, the assembly macros just need to be
generalized slightly to handle additional asm primitives.

I started this fix some time ago [4] by optimizing the infrastructure
first that could relax some register pressure, but there was no
reaction from x86 maintainers. I don't want to waste my time on
patches that get ignored, so I dropped the ball.

[4] https://lore.kernel.org/lkml/20240605181424.3228-1-ubizjak@gmail.com/

So, without some interest from x86 maintainers, the issue will remain
unfixed. If they want x86_32 arch to die then inline locking
primitives involving cmpxchg8b are one of the nails in its coffin.

Uros.