linux-kernel - [PATCH v3] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86

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Date:   Mon, 12 Sep 2022 16:39:03 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     x86@...nel.org, linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH v3] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86_CMPXCHG64

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_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>

Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: "H. Peter Anvin" <hpa@...or.com>
Signed-off-by: Uros Bizjak <ubizjak@...il.com>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
---
v3:
 - Update commit message to refer to lockref_get_not_zero
   instead of removed lockref_get_or_lock
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 f9920f1341c8..93c8e6cf1cff 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
@@ -110,6 +109,7 @@ 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
-- 
2.37.2