linux-kernel - [RFC][PATCH 4/5] atomic,x86: Implement atomic_dec_and_test

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Date:   Wed, 08 Dec 2021 19:36:59 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     will@...nel.org, boqun.feng@...il.com
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, peterz@...radead.org,
        mark.rutland@....com, elver@...gle.com, keescook@...omium.org,
        hch@...radead.org, torvalds@...ux-foundation.org
Subject: [RFC][PATCH 4/5] atomic,x86: Implement atomic_dec_and_test_ofl()

Provide a better implementation of atomic_{inc,dec_and_test}_ofl() by
making use of the atomic-op condition codes.

This further improves the fast path code:

    a980:       b8 ff ff ff ff          mov    $0xffffffff,%eax
    a985:       f0 0f c1 07             lock xadd %eax,(%rdi)
    a989:       83 e8 01                sub    $0x1,%eax
    a98c:       78 20                   js     a9ae <ring_buffer_put+0x2e>
    a98e:       74 01                   je     a991 <ring_buffer_put+0x11>
    a990:       c3                      ret

to:

    ab81:       48 89 fb                mov    %rdi,%rbx
    ab84:       f0 ff 0f                lock decl (%rdi)
    ab87:       7c 04                   jl     ab8d <ring_buffer_put+0xd>
    ab89:       74 10                   je     ab9b <ring_buffer_put+0x1b>
    ab8b:       c3                      ret

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/include/asm/atomic.h |   23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

--- a/arch/x86/include/asm/atomic.h
+++ b/arch/x86/include/asm/atomic.h
@@ -263,6 +263,29 @@ static __always_inline int arch_atomic_f
 }
 #define arch_atomic_fetch_xor arch_atomic_fetch_xor
 
+#define arch_atomic_dec_ofl(_v, _label)					\
+	asm_volatile_goto(LOCK_PREFIX "decl %[var]\n\t"			\
+			  "jle %l1"					\
+			  : : [var] "m" ((_v)->counter)			\
+			  : "memory"					\
+			  : _label)
+
+#define arch_atomic_dec_and_test_ofl(_v, _label)			\
+({									\
+	__label__ __zero;						\
+	__label__ __out;						\
+	bool __ret = false;						\
+	asm_volatile_goto(LOCK_PREFIX "decl %[var]\n\t"			\
+			  "jl %l2\n\t"					\
+			  "je %l[__zero]"				\
+			  : : [var] "m" ((_v)->counter)			\
+			  : "memory"					\
+			  : __zero, _label);				\
+	goto __out;							\
+__zero:	__ret = true;							\
+__out:	__ret;								\
+})
+
 #ifdef CONFIG_X86_32
 # include <asm/atomic64_32.h>
 #else