扫描标记与标记辅助

func gcBgMarkWorker(_p_ *p) {
    gp := getg()

    type parkInfo struct {
        m      muintptr // Release this m on park.
        attach puintptr // If non-nil, attach to this p on park.
    }
    // We pass park to a gopark unlock function, so it can't be on
    // the stack (see gopark). Prevent deadlock from recursively
    // starting GC by disabling preemption.
    gp.m.preemptoff = "GC worker init"
    park := new(parkInfo)
    gp.m.preemptoff = ""

    park.m.set(acquirem())
    park.attach.set(_p_)
    // Inform gcBgMarkStartWorkers that this worker is ready.
    // After this point, the background mark worker is scheduled
    // cooperatively by gcController.findRunnable. Hence, it must
    // never be preempted, as this would put it into _Grunnable
    // and put it on a run queue. Instead, when the preempt flag
    // is set, this puts itself into _Gwaiting to be woken up by
    // gcController.findRunnable at the appropriate time.
    notewakeup(&work.bgMarkReady)

    for {
        // Go to sleep until woken by gcController.findRunnable.
        // We can't releasem yet since even the call to gopark
        // may be preempted.
        gopark(func(g *g, parkp unsafe.Pointer) bool {
            park := (*parkInfo)(parkp)

            // The worker G is no longer running, so it's
            // now safe to allow preemption.
            releasem(park.m.ptr())

            // If the worker isn't attached to its P,
            // attach now. During initialization and after
            // a phase change, the worker may have been
            // running on a different P. As soon as we
            // attach, the owner P may schedule the
            // worker, so this must be done after the G is
            // stopped.
            if park.attach != 0 {
                p := park.attach.ptr()
                park.attach.set(nil)
                // cas the worker because we may be
                // racing with a new worker starting
                // on this P.
                if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) {
                    // The P got a new worker.
                    // Exit this worker.
                    return false
                }
            }
            return true
        }, unsafe.Pointer(park), waitReasonGCWorkerIdle, traceEvGoBlock, 0)

        // Loop until the P dies and disassociates this
        // worker (the P may later be reused, in which case
        // it will get a new worker) or we failed to associate.
        if _p_.gcBgMarkWorker.ptr() != gp {
            break
        }

        // Disable preemption so we can use the gcw. If the
        // scheduler wants to preempt us, we'll stop draining,
        // dispose the gcw, and then preempt.
        park.m.set(acquirem())
        ...

        startTime := nanotime()
        _p_.gcMarkWorkerStartTime = startTime

        decnwait := atomic.Xadd(&work.nwait, -1)
        ...

        systemstack(func() {
            // Mark our goroutine preemptible so its stack
            // can be scanned. This lets two mark workers
            // scan each other (otherwise, they would
            // deadlock). We must not modify anything on
            // the G stack. However, stack shrinking is
            // disabled for mark workers, so it is safe to
            // read from the G stack.
            casgstatus(gp, _Grunning, _Gwaiting)
            switch _p_.gcMarkWorkerMode {
            ...
            case gcMarkWorkerDedicatedMode:
                gcDrain(&_p_.gcw, gcDrainUntilPreempt|gcDrainFlushBgCredit)
                if gp.preempt {
                    // We were preempted. This is
                    // a useful signal to kick
                    // everything out of the run
                    // queue so it can run
                    // somewhere else.
                    lock(&sched.lock)
                    for {
                        gp, _ := runqget(_p_)
                        if gp == nil {
                            break
                        }
                        globrunqput(gp)
                    }
                    unlock(&sched.lock)
                }
                // Go back to draining, this time
                // without preemption.
                gcDrain(&_p_.gcw, gcDrainFlushBgCredit)
            case gcMarkWorkerFractionalMode:
                gcDrain(&_p_.gcw, gcDrainFractional|gcDrainUntilPreempt|gcDrainFlushBgCredit)
            case gcMarkWorkerIdleMode:
                gcDrain(&_p_.gcw, gcDrainIdle|gcDrainUntilPreempt|gcDrainFlushBgCredit)
            }
            casgstatus(gp, _Gwaiting, _Grunning)
        })

        // Account for time.
        duration := nanotime() - startTime
        switch _p_.gcMarkWorkerMode {
        case gcMarkWorkerDedicatedMode:
            atomic.Xaddint64(&gcController.dedicatedMarkTime, duration)
            atomic.Xaddint64(&gcController.dedicatedMarkWorkersNeeded, 1)
        case gcMarkWorkerFractionalMode:
            atomic.Xaddint64(&gcController.fractionalMarkTime, duration)
            atomic.Xaddint64(&_p_.gcFractionalMarkTime, duration)
        case gcMarkWorkerIdleMode:
            atomic.Xaddint64(&gcController.idleMarkTime, duration)
        }

        // Was this the last worker and did we run out
        // of work?
        incnwait := atomic.Xadd(&work.nwait, +1)
        if incnwait > work.nproc {
            ...
        }

        // If this worker reached a background mark completion
        // point, signal the main GC goroutine.
        if incnwait == work.nproc && !gcMarkWorkAvailable(nil) {
            // Make this G preemptible and disassociate it
            // as the worker for this P so
            // findRunnableGCWorker doesn't try to
            // schedule it.
            _p_.gcBgMarkWorker.set(nil)
            releasem(park.m.ptr())

            gcMarkDone()

            // Disable preemption and prepare to reattach
            // to the P.
            //
            // We may be running on a different P at this
            // point, so we can't reattach until this G is
            // parked.
            park.m.set(acquirem())
            park.attach.set(_p_)
        }
    }
}

进一步阅读的参考文献