From 50cabf1d9505e31754cf1af2ff3ca6d137f4a3a3 Mon Sep 17 00:00:00 2001
From: Achilleas Anagnostopoulos <achilleas.imap.mbox@gmail.com>
Date: Fri, 15 Jun 2018 22:59:02 +0100
Subject: [PATCH] sync: implement spinlock primitive

---
 src/gopheros/kernel/sync/spinlock.go      | 38 +++++++++++++++++++++
 src/gopheros/kernel/sync/spinlock_amd64.s | 41 +++++++++++++++++++++++
 src/gopheros/kernel/sync/spinlock_test.go | 39 +++++++++++++++++++++
 3 files changed, 118 insertions(+)
 create mode 100644 src/gopheros/kernel/sync/spinlock.go
 create mode 100644 src/gopheros/kernel/sync/spinlock_amd64.s
 create mode 100644 src/gopheros/kernel/sync/spinlock_test.go

diff --git a/src/gopheros/kernel/sync/spinlock.go b/src/gopheros/kernel/sync/spinlock.go
new file mode 100644
index 0000000..2759e2f
--- /dev/null
+++ b/src/gopheros/kernel/sync/spinlock.go
@@ -0,0 +1,38 @@
+// Package sync provides synchronization primitive implementations for spinlocks
+// and semaphore.
+package sync
+
+import "sync/atomic"
+
+var (
+	// TODO: replace with real yield function when context-switching is implemented.
+	yieldFn func()
+)
+
+// Spinlock implements a lock where each task trying to acquire it busy-waits
+// till the lock becomes available.
+type Spinlock struct {
+	state uint32
+}
+
+// Acquire blocks until the lock can be acquired by the currently active task.
+// Any attempt to re-acquire a lock already held by the current task will cause
+// a deadlock.
+func (l *Spinlock) Acquire() {
+	archAcquireSpinlock(&l.state, 1)
+}
+
+// TryToAcquire attempts to acquire the lock and returns true if the lock could
+// be acquired or false otherwise.
+func (l *Spinlock) TryToAcquire() bool {
+	return atomic.SwapUint32(&l.state, 1) == 0
+}
+
+// Release relinquishes a held lock allowing other tasks to acquire it. Calling
+// Release while the lock is free has no effect.
+func (l *Spinlock) Release() {
+	atomic.StoreUint32(&l.state, 0)
+}
+
+// archAcquireSpinlock is an arch-specific implementation for acquiring the lock.
+func archAcquireSpinlock(state *uint32, attemptsBeforeYielding uint32)
diff --git a/src/gopheros/kernel/sync/spinlock_amd64.s b/src/gopheros/kernel/sync/spinlock_amd64.s
new file mode 100644
index 0000000..634066e
--- /dev/null
+++ b/src/gopheros/kernel/sync/spinlock_amd64.s
@@ -0,0 +1,41 @@
+#include "textflag.h"
+
+TEXT ·archAcquireSpinlock(SB),NOSPLIT,$0-12
+	MOVQ state+0(FP), AX
+	MOVL attemptsBeforeYielding+8(FP), CX
+
+try_acquire:
+	MOVL $1, BX
+	XCHGL 0(AX), BX
+	TESTL BX, BX
+	JNZ spin
+
+	// Lock succesfully acquired 
+	RET
+
+spin:
+	// Send hint to the CPU that we are in a spinlock loop
+	PAUSE
+
+	// Do a dirty read to check the state and try to acquire the lock 
+	// once we detect it is free 
+	MOVL 0(AX), BX
+	TESTL BX, BX
+	JZ try_acquire
+
+	// Keep retrying till we exceed attemptsBeforeYielding; this allows us 
+	// to grab the lock if a task on another CPU releases the lock while we 
+	// spin.
+	DECL CX
+	JNZ spin
+
+	// Yield (if yieldFn is set) and spin again
+	MOVQ ·yieldFn+0(SB), AX
+	TESTQ AX, AX
+	JZ replenish_attempt_counter
+	CALL 0(AX)
+
+replenish_attempt_counter:
+	MOVQ state+0(FP), AX
+	MOVL attemptsBeforeYielding+8(FP), CX
+	JMP spin
diff --git a/src/gopheros/kernel/sync/spinlock_test.go b/src/gopheros/kernel/sync/spinlock_test.go
new file mode 100644
index 0000000..39286e4
--- /dev/null
+++ b/src/gopheros/kernel/sync/spinlock_test.go
@@ -0,0 +1,39 @@
+package sync
+
+import (
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
+
+func TestSpinlock(t *testing.T) {
+	// Substitute the yieldFn with runtime.Gosched to avoid deadlocks while testing
+	defer func(origYieldFn func()) { yieldFn = origYieldFn }(yieldFn)
+	yieldFn = runtime.Gosched
+
+	var (
+		sl         Spinlock
+		wg         sync.WaitGroup
+		numWorkers = 10
+	)
+
+	sl.Acquire()
+
+	if sl.TryToAcquire() != false {
+		t.Error("expected TryToAcquire to return false when lock is held")
+	}
+
+	wg.Add(numWorkers)
+	for i := 0; i < numWorkers; i++ {
+		go func(worker int) {
+			sl.Acquire()
+			sl.Release()
+			wg.Done()
+		}(i)
+	}
+
+	<-time.After(100 * time.Millisecond)
+	sl.Release()
+	wg.Wait()
+}