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Date: Wed, 26 Jun 2024 09:39:39 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org, x86@...nel.org,
kernel test robot <lkp@...el.com>
Subject: Re: [PATCH] x86-32: fix cmpxchg8b_emu build error with clang
On Wed, Jun 26, 2024 at 3:13 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> The kernel test robot reported that clang no longer compiles the 32-bit
> x86 kernel in some configurations due to commit 95ece48165c1
> ("locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}()
> functions").
>
> The build fails with
>
> arch/x86/include/asm/cmpxchg_32.h:149:9: error: inline assembly requires more registers than available
>
> and the reason seems to be that not only does the cmpxchg8b instruction
> need four fixed registers (EDX:EAX and ECX:EBX), with the emulation
> fallback the inline asm also wants a fifth fixed register for the
> address (it uses %esi for that, but that's just a software convention
> with cmpxchg8b_emu).
>
> Avoiding using another pointer input to the asm (and just forcing it to
> use the "0(%esi)" addressing that we end up requiring for the sw
> fallback) seems to fix the issue.
>
> Reported-by: kernel test robot <lkp@...el.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202406230912.F6XFIyA6-lkp@intel.com/
> Fixes: 95ece48165c1 ("locking/atomic/x86: Rewrite x86_32 arch_atomic64_{,fetch}_{and,or,xor}() functions")
> Link: https://lore.kernel.org/all/202406230912.F6XFIyA6-lkp@intel.com/
> Suggested-by: Uros Bizjak <ubizjak@...il.com>
> Signed-off-by: Linus Torvalds <torvalds@...ux-foundation.org>
> ---
>
> Added commit message, and updated the asm to use '%a[ptr]' instead of
> writing out the addressing by hand.
>
> Still doing the 'oldp' writeback unconmditionally. The code generation
> for the case I checked were the same for both clang and gcc, but until
> Uros hits me with the big clue-hammer, I think it's the simpler code
> that leaves room for potentially better optimizations too.
You probably want to look at 44fe84459faf1 ("locking/atomic: Fix
atomic_try_cmpxchg() semantics") [1] and the long LKML discussion at
[2].
--quote--
This code is broken with the current implementation, the problem is
with unconditional update of *__po.
In case of success it writes the same value back into *__po, but in
case of cmpxchg success we might have lose ownership of some memory
locations and potentially over what __po has pointed to. The same
holds for the re-read of *__po. "
--/quote--
[1] https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/commit/?id=44fe84459faf1a7781595b7c64cd36daf2f2827d
[2] https://lore.kernel.org/lkml/CACT4Y+bG+a0w6j6v1AmBE7fqqMSPyPEm4QimCzCouicmHT8FqA@mail.gmail.com/
Uros.
>
> This falls solidly in the "looks ok to me, but still untested" category
> for me. It fixes the clang build issue in my build testing, but I no
> longer have a 32-bit test environment, so no actual runtime testing.
>
> arch/x86/include/asm/cmpxchg_32.h | 28 ++++++++++++----------------
> 1 file changed, 12 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
> index ed2797f132ce..4444a8292c7a 100644
> --- a/arch/x86/include/asm/cmpxchg_32.h
> +++ b/arch/x86/include/asm/cmpxchg_32.h
> @@ -88,18 +88,17 @@ static __always_inline bool __try_cmpxchg64_local(volatile u64 *ptr, u64 *oldp,
>
> #define __arch_cmpxchg64_emu(_ptr, _old, _new, _lock_loc, _lock) \
> ({ \
> - union __u64_halves o = { .full = (_old), }, \
> - n = { .full = (_new), }; \
> + __u64 o = (_old); \
> + union __u64_halves n = { .full = (_new), }; \
> \
> asm volatile(ALTERNATIVE(_lock_loc \
> "call cmpxchg8b_emu", \
> - _lock "cmpxchg8b %[ptr]", X86_FEATURE_CX8) \
> - : [ptr] "+m" (*(_ptr)), \
> - "+a" (o.low), "+d" (o.high) \
> - : "b" (n.low), "c" (n.high), "S" (_ptr) \
> + _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
> + : "+A" (o) \
> + : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
> : "memory"); \
> \
> - o.full; \
> + o; \
> })
>
> static __always_inline u64 arch_cmpxchg64(volatile u64 *ptr, u64 old, u64 new)
> @@ -116,22 +115,19 @@ static __always_inline u64 arch_cmpxchg64_local(volatile u64 *ptr, u64 old, u64
>
> #define __arch_try_cmpxchg64_emu(_ptr, _oldp, _new, _lock_loc, _lock) \
> ({ \
> - union __u64_halves o = { .full = *(_oldp), }, \
> - n = { .full = (_new), }; \
> + __u64 o = *(_oldp); \
> + union __u64_halves n = { .full = (_new), }; \
> bool ret; \
> \
> asm volatile(ALTERNATIVE(_lock_loc \
> "call cmpxchg8b_emu", \
> - _lock "cmpxchg8b %[ptr]", X86_FEATURE_CX8) \
> + _lock "cmpxchg8b %a[ptr]", X86_FEATURE_CX8) \
> CC_SET(e) \
> - : CC_OUT(e) (ret), \
> - [ptr] "+m" (*(_ptr)), \
> - "+a" (o.low), "+d" (o.high) \
> - : "b" (n.low), "c" (n.high), "S" (_ptr) \
> + : CC_OUT(e) (ret), "+A" (o) \
> + : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr) \
> : "memory"); \
> \
> - if (unlikely(!ret)) \
> - *(_oldp) = o.full; \
> + *(_oldp) = o; \
> \
> likely(ret); \
> })
> --
> 2.45.1.209.gc6f12300df
>
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