Question

从空闲链表提取 M，或放回。

// runtime2.go

type schedt struct {
    midle   muintptr     // idle m's waiting for work
    nmidle  int32        // number of idle m's waiting for work
}

// proc.go

// Try to get an m from midle list.

func mget() *m {
    mp := sched.midle.ptr()
    if mp != nil {
        sched.midle = mp.schedlink
        sched.nmidle--
    }
    
    return mp
}

// Put mp on midle list.

func mput(mp *m) {
    mp.schedlink = sched.midle
    sched.midle.set(mp)
    sched.nmidle++
    
    // 统计正在运行 M 数量，检查是否死锁（deadlock）。
    checkdead()
}