code review 4711045: runtime: improve Linux mutex

src/pkg/runtime/amd64/asm.s

 // Copyright 2009 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 #include "amd64/asm.h"
 
 TEXT _rt0_amd64(SB),7,$-8
         // copy arguments forward on an even stack
         MOVQ    0(DI), AX               // argc
         LEAQ    8(DI), BX               // argv
@@ skipping 358 matching lines @@
 // Atomically:
 //      *val += delta;
 //      return *val;
 TEXT runtime·xadd(SB), 7, $0
         MOVQ    8(SP), BX
         MOVL    16(SP), AX
         MOVL    AX, CX
         LOCK
         XADDL   AX, 0(BX)
         ADDL    CX, AX
+        RET
+
+TEXT runtime·xchg(SB), 7, $0
+        MOVQ    8(SP), BX
+        MOVL    16(SP), AX
+        XCHGL   AX, 0(BX)
+        RET
+
+TEXT runtime·procyield(SB),7,$0
+        MOVL    8(SP), AX
+again:
+        PAUSE
+        SUBL    $1, AX
+        JNZ     again
         RET
 
 TEXT runtime·atomicstorep(SB), 7, $0
         MOVQ    8(SP), BX
         MOVQ    16(SP), AX
         XCHGQ   AX, 0(BX)
         RET
 
 // void jmpdefer(fn, sp);
 // called from deferreturn.
@@ skipping 159 matching lines @@
         MOVQ    x+0(FP),AX              // addr of first arg
         MOVQ    x+8(FP), BX
         MOVQ    BX, -8(AX)              // set calling pc
         RET
 
 TEXT runtime·getcallersp(SB),7,$0
         MOVQ    sp+0(FP), AX
         RET
 
 GLOBL runtime·tls0(SB), $64