[PATCH v2 RESEND] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_CMPXCHG64

From: Uros Bizjak
Date: Sun Jun 26 2022 - 16:18:09 EST


Commit bc08b449ee14ace4d869adaa1bb35a44ce68d775 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 CMPXCHG_LOOP macro, the compiler actually produces sane code,
improving lockref_get_or_lock main loop from:

2a5: 8d 48 01 lea 0x1(%eax),%ecx
2a8: 85 c0 test %eax,%eax
2aa: 7e 3c jle 2e8 <lockref_get_or_lock+0x78>
2ac: 8b 44 24 08 mov 0x8(%esp),%eax
2b0: 8b 54 24 0c mov 0xc(%esp),%edx
2b4: 8b 74 24 04 mov 0x4(%esp),%esi
2b8: f0 0f c7 0e lock cmpxchg8b (%esi)
2bc: 8b 4c 24 0c mov 0xc(%esp),%ecx
2c0: 89 c3 mov %eax,%ebx
2c2: 89 d0 mov %edx,%eax
2c4: 8b 74 24 08 mov 0x8(%esp),%esi
2c8: 31 ca xor %ecx,%edx
2ca: 31 de xor %ebx,%esi
2cc: 09 f2 or %esi,%edx
2ce: 75 40 jne 310 <lockref_get_or_lock+0xa0>

to:

2d: 8d 4f 01 lea 0x1(%edi),%ecx
30: 85 ff test %edi,%edi
32: 7e 1c jle 50 <lockref_get_or_lock+0x50>
34: f0 0f c7 0e lock cmpxchg8b (%esi)
38: 75 36 jne 70 <lockref_get_or_lock+0x70>

Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
v2:
- select ARCH_USE_CMPXCHG_LOCKREF for CONFIG_X86_CMPXCHG which
is unconditionally defined for X86_64
---
arch/x86/Kconfig | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index be0b95e51df6..22555e0c894d 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -27,7 +27,6 @@ config X86_64
# Options that are inherently 64-bit kernel only:
select ARCH_HAS_GIGANTIC_PAGE
select ARCH_SUPPORTS_INT128 if CC_HAS_INT128
- select ARCH_USE_CMPXCHG_LOCKREF
select HAVE_ARCH_SOFT_DIRTY
select MODULES_USE_ELF_RELA
select NEED_DMA_MAP_STATE
@@ -111,6 +110,7 @@ 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
--
2.35.3