Commit 76932725 authored Jun 25, 2024 by Linus Torvalds

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 95ece481
("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@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202406230912.F6XFIyA6-lkp@intel.com/
Fixes: 95ece481 ("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@gmail.com>
Reviewed-and-Tested-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 84dd4373

arch/x86/include/asm/cmpxchg_32.h

+5 −7

Original line number	Diff line number	Diff line
		@@ -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)) \