linux-kernel - [PATCH v2] x86-32: fix cmpxchg8b

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Date: Wed, 26 Jun 2024 08:11:32 -0700
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Uros Bizjak <ubizjak@...il.com>,
	Ingo Molnar <mingo@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org,
	x86@...nel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	kernel test robot <lkp@...el.com>
Subject: [PATCH v2] x86-32: fix cmpxchg8b_emu build error with clang

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>
---

This is the minimal patch, with the other simplifications removed.

I still think that the case where somebody passes in a "oldp" that can
change during the cmpxchg is entirely broken, and is not actually valid. 

If it was valid, we'd have to use READ_ONCE() and WRITE_ONCE() in the
"oldp" updates anyway just to make verification tools happy. We don't.

But hey, that simplification doesn't matter for fixing this build issue,
so let's keep the change minimal. 

 arch/x86/include/asm/cmpxchg_32.h | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/cmpxchg_32.h b/arch/x86/include/asm/cmpxchg_32.h
index ed2797f132ce..62cef2113ca7 100644
--- a/arch/x86/include/asm/cmpxchg_32.h
+++ b/arch/x86/include/asm/cmpxchg_32.h
@@ -93,10 +93,9 @@ static __always_inline bool __try_cmpxchg64_local(volatile u64 *ptr, u64 *oldp,
 									\
 	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.low), "+d" (o.high)			\
+		     : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr)	\
 		     : "memory");					\
 									\
 	o.full;								\
@@ -122,12 +121,11 @@ static __always_inline u64 arch_cmpxchg64_local(volatile u64 *ptr, u64 old, u64
 									\
 	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)		\
+		     : "b" (n.low), "c" (n.high), [ptr] "S" (_ptr)	\
 		     : "memory");					\
 									\
 	if (unlikely(!ret))						\
-- 
2.45.1.209.gc6f12300df