locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_CMPXCHG64

The following commit:

  bc08b449ee14 ("lockref: implement lockless reference count updates using cmpxchg()")

enabled lockless reference count updates using cmpxchg() only for x86_64,
and left x86_32 behind due to inability to detect support for
cmpxchg8b instruction.

Nowadays, we can use CONFIG_X86_CMPXCHG64 for this purpose. Also,
by using try_cmpxchg64() instead of cmpxchg64() in the CMPXCHG_LOOP macro,
the compiler actually produces sane code, improving the
lockref_get_not_zero() main loop from:

  eb:	8d 48 01             	lea    0x1(%eax),%ecx
  ee:	85 c0                	test   %eax,%eax
  f0:	7e 2f                	jle    121 <lockref_get_not_zero+0x71>
  f2:	8b 44 24 10          	mov    0x10(%esp),%eax
  f6:	8b 54 24 14          	mov    0x14(%esp),%edx
  fa:	8b 74 24 08          	mov    0x8(%esp),%esi
  fe:	f0 0f c7 0e          	lock cmpxchg8b (%esi)
 102:	8b 7c 24 14          	mov    0x14(%esp),%edi
 106:	89 c1                	mov    %eax,%ecx
 108:	89 c3                	mov    %eax,%ebx
 10a:	8b 74 24 10          	mov    0x10(%esp),%esi
 10e:	89 d0                	mov    %edx,%eax
 110:	31 fa                	xor    %edi,%edx
 112:	31 ce                	xor    %ecx,%esi
 114:	09 f2                	or     %esi,%edx
 116:	75 58                	jne    170 <lockref_get_not_zero+0xc0>

to:

 350:	8d 4f 01             	lea    0x1(%edi),%ecx
 353:	85 ff                	test   %edi,%edi
 355:	7e 79                	jle    3d0 <lockref_get_not_zero+0xb0>
 357:	f0 0f c7 0e          	lock cmpxchg8b (%esi)
 35b:	75 53                	jne    3b0 <lockref_get_not_zero+0x90>

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/20230918184050.9180-1-ubizjak@gmail.com

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 66bfabae8814919c61902da3cdc77a45d9efa775..1379603016fd95a2b389547ff2d49eb0b0fc543b 100644 (file)
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -28,7 +28,6 @@ config X86_64
        select ARCH_HAS_GIGANTIC_PAGE
        select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
        select ARCH_SUPPORTS_PER_VMA_LOCK
-       select ARCH_USE_CMPXCHG_LOCKREF
        select HAVE_ARCH_SOFT_DIRTY
        select MODULES_USE_ELF_RELA
        select NEED_DMA_MAP_STATE
@@ -118,6 +117,7 @@ config X86
        select ARCH_SUPPORTS_LTO_CLANG
        select ARCH_SUPPORTS_LTO_CLANG_THIN
        select ARCH_USE_BUILTIN_BSWAP
+       select ARCH_USE_CMPXCHG_LOCKREF         if X86_CMPXCHG64
        select ARCH_USE_MEMTEST
        select ARCH_USE_QUEUED_RWLOCKS
        select ARCH_USE_QUEUED_SPINLOCKS