go语言源码赏析3(锁)

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func cansemacquire(addr *uint32) bool {
	for {
		v := atomic.Load(addr)
		if v == 0 {
			return false
		}
		if atomic.Cas(addr, v, v-1) {
			return true
		}
	}
}

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func semrelease1(addr *uint32, handoff bool, skipframes int) {
	root := semroot(addr)
	atomic.Xadd(addr, 1)

	// Easy case: no waiters?
	// This check must happen after the xadd, to avoid a missed wakeup
	// (see loop in semacquire).
	if atomic.Load(&root.nwait) == 0 {
		return
	}

	// Harder case: search for a waiter and wake it.
	lockWithRank(&root.lock, lockRankRoot)
	if atomic.Load(&root.nwait) == 0 {
		// The count is already consumed by another goroutine,
		// so no need to wake up another goroutine.
		unlock(&root.lock)
		return
	}
	s, t0 := root.dequeue(addr)
	if s != nil {
		atomic.Xadd(&root.nwait, -1)
	}
	unlock(&root.lock)
	if s != nil { // May be slow or even yield, so unlock first
		acquiretime := s.acquiretime
		if acquiretime != 0 {
			mutexevent(t0-acquiretime, 3+skipframes)
		}
		if s.ticket != 0 {
			throw("corrupted semaphore ticket")
		}
		if handoff && cansemacquire(addr) {
			s.ticket = 1
		}
		readyWithTime(s, 5+skipframes)
		if s.ticket == 1 && getg().m.locks == 0 {
			// Direct G handoff
			// readyWithTime has added the waiter G as runnext in the
			// current P; we now call the scheduler so that we start running
			// the waiter G immediately.
			// Note that waiter inherits our time slice: this is desirable
			// to avoid having a highly contended semaphore hog the P
			// indefinitely. goyield is like Gosched, but it emits a
			// "preempted" trace event instead and, more importantly, puts
			// the current G on the local runq instead of the global one.
			// We only do this in the starving regime (handoff=true), as in
			// the non-starving case it is possible for a different waiter
			// to acquire the semaphore while we are yielding/scheduling,
			// and this would be wasteful. We wait instead to enter starving
			// regime, and then we start to do direct handoffs of ticket and
			// P.
			// See issue 33747 for discussion.
			goyield()
		}
	}
}

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type semaRoot struct {
	lock  mutex
	treap *sudog // root of balanced tree of unique waiters.
	nwait uint32 // Number of waiters. Read w/o the lock.
}

// leaseSudog to allocate and free them.
type sudog struct {
	// The following fields are protected by the hchan.lock of the
	// channel this sudog is blocking on. shrinkstack depends on
	// this for sudogs involved in channel ops.

	g *g

	next *sudog
	prev *sudog
	elem unsafe.Pointer // data element (may point to stack)

	// The following fields are never accessed concurrently.
	// For channels, waitlink is only accessed by g.
	// For semaphores, all fields (including the ones above)
	// are only accessed when holding a semaRoot lock.

	acquiretime int64
	releasetime int64
	ticket      uint32

	// isSelect indicates g is participating in a select, so
	// g.selectDone must be CAS'd to win the wake-up race.
	isSelect bool

	// success indicates whether communication over channel c
	// succeeded. It is true if the goroutine was awoken because a
	// value was delivered over channel c, and false if awoken
	// because c was closed.
	success bool

	parent   *sudog // semaRoot binary tree
	waitlink *sudog // g.waiting list or semaRoot
	waittail *sudog // semaRoot
	c        *hchan // channel
}