linux-kernel - [tip: locking/core] locking/lockref/x86: Enable ARCH_USE_CMPXCHG_LOCKREF for X86

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Date:   Mon, 18 Sep 2023 20:27:47 -0000
From:   "tip-bot2 for Uros Bizjak" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>, Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: locking/core] locking/lockref/x86: Enable
 ARCH_USE_CMPXCHG_LOCKREF for X86_CMPXCHG64

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     a432b7c0cf420dbf2448c6bda6a6697afbb153d5
Gitweb:        https://git.kernel.org/tip/a432b7c0cf420dbf2448c6bda6a6697afbb153d5
Author:        Uros Bizjak <ubizjak@...il.com>
AuthorDate:    Mon, 18 Sep 2023 20:40:27 +02:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 18 Sep 2023 22:18:32 +02:00

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@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Linus Torvalds <torvalds@...ux-foundation.org>
Link: https://lore.kernel.org/r/20230918184050.9180-1-ubizjak@gmail.com
---
 arch/x86/Kconfig | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 66bfaba..1379603 100644
--- 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