深入理解 GMP

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// 声明 channel
var ch chan T	// 双向 channel
var ch chan<- T	// 只能发送 msg 的 channel
var ch <-chan T 	// 只能接收 msg 的 channel

// 创建 channel
ch := make(chan T, capicity)	// 双向 channel
ch := make(chan<- T, capicity)	// 只能发送 msg 的 channel
ch := make(<-chan T, capicity)	// 只能接收 msg 的 channel

// 访问 channel
ch <- msg	// 发送 msg
msg := <-ch	// 接收 msg
msg, ok := <-ch // 接收 msg，同时判断 channel 是否接收成功
close(ch)	// 关闭 channel

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type g struct {
	// Stack parameters.
	// stack describes the actual stack memory: [stack.lo, stack.hi).
	// stackguard0 is the stack pointer compared in the Go stack growth prologue.
	// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
	// stackguard1 is the stack pointer compared in the //go:systemstack stack growth prologue.
	// It is stack.lo+StackGuard on g0 and gsignal stacks.
	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
	stack       stack   // offset known to runtime/cgo
	stackguard0 uintptr // offset known to liblink
	stackguard1 uintptr // offset known to liblink

	_panic    *_panic // innermost panic - offset known to liblink
	_defer    *_defer // innermost defer
	m         *m      // current m; offset known to arm liblink
	sched     gobuf
	syscallsp uintptr // if status==Gsyscall, syscallsp = sched.sp to use during gc
	syscallpc uintptr // if status==Gsyscall, syscallpc = sched.pc to use during gc
	stktopsp  uintptr // expected sp at top of stack, to check in traceback
	// param is a generic pointer parameter field used to pass
	// values in particular contexts where other storage for the
	// parameter would be difficult to find. It is currently used
	// in four ways:
	// 1. When a channel operation wakes up a blocked goroutine, it sets param to
	//    point to the sudog of the completed blocking operation.
	// 2. By gcAssistAlloc1 to signal back to its caller that the goroutine completed
	//    the GC cycle. It is unsafe to do so in any other way, because the goroutine's
	//    stack may have moved in the meantime.
	// 3. By debugCallWrap to pass parameters to a new goroutine because allocating a
	//    closure in the runtime is forbidden.
	// 4. When a panic is recovered and control returns to the respective frame,
	//    param may point to a savedOpenDeferState.
	param        unsafe.Pointer
	atomicstatus atomic.Uint32
	stackLock    uint32 // sigprof/scang lock; TODO: fold in to atomicstatus
	goid         uint64
	schedlink    guintptr
	waitsince    int64      // approx time when the g become blocked
	waitreason   waitReason // if status==Gwaiting

	preempt       bool // preemption signal, duplicates stackguard0 = stackpreempt
	preemptStop   bool // transition to _Gpreempted on preemption; otherwise, just deschedule
	preemptShrink bool // shrink stack at synchronous safe point

	// asyncSafePoint is set if g is stopped at an asynchronous
	// safe point. This means there are frames on the stack
	// without precise pointer information.
	asyncSafePoint bool

	paniconfault bool // panic (instead of crash) on unexpected fault address
	gcscandone   bool // g has scanned stack; protected by _Gscan bit in status
	throwsplit   bool // must not split stack
	// activeStackChans indicates that there are unlocked channels
	// pointing into this goroutine's stack. If true, stack
	// copying needs to acquire channel locks to protect these
	// areas of the stack.
	activeStackChans bool
	// parkingOnChan indicates that the goroutine is about to
	// park on a chansend or chanrecv. Used to signal an unsafe point
	// for stack shrinking.
	parkingOnChan atomic.Bool
	// inMarkAssist indicates whether the goroutine is in mark assist.
	// Used by the execution tracer.
	inMarkAssist bool
	coroexit     bool // argument to coroswitch_m

	raceignore    int8  // ignore race detection events
	nocgocallback bool  // whether disable callback from C
	tracking      bool  // whether we're tracking this G for sched latency statistics
	trackingSeq   uint8 // used to decide whether to track this G
	trackingStamp int64 // timestamp of when the G last started being tracked
	runnableTime  int64 // the amount of time spent runnable, cleared when running, only used when tracking
	lockedm       muintptr
	sig           uint32
	writebuf      []byte
	sigcode0      uintptr
	sigcode1      uintptr
	sigpc         uintptr
	parentGoid    uint64          // goid of goroutine that created this goroutine
	gopc          uintptr         // pc of go statement that created this goroutine
	ancestors     *[]ancestorInfo // ancestor information goroutine(s) that created this goroutine (only used if debug.tracebackancestors)
	startpc       uintptr         // pc of goroutine function
	racectx       uintptr
	waiting       *sudog         // sudog structures this g is waiting on (that have a valid elem ptr); in lock order
	cgoCtxt       []uintptr      // cgo traceback context
	labels        unsafe.Pointer // profiler labels
	timer         *timer         // cached timer for time.Sleep
	selectDone    atomic.Uint32  // are we participating in a select and did someone win the race?

	coroarg *coro // argument during coroutine transfers

	// goroutineProfiled indicates the status of this goroutine's stack for the
	// current in-progress goroutine profile
	goroutineProfiled goroutineProfileStateHolder

	// Per-G tracer state.
	trace gTraceState

	// Per-G GC state

	// gcAssistBytes is this G's GC assist credit in terms of
	// bytes allocated. If this is positive, then the G has credit
	// to allocate gcAssistBytes bytes without assisting. If this
	// is negative, then the G must correct this by performing
	// scan work. We track this in bytes to make it fast to update
	// and check for debt in the malloc hot path. The assist ratio
	// determines how this corresponds to scan work debt.
	gcAssistBytes int64
}

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type m struct {
	g0      *g     // goroutine with scheduling stack
	morebuf gobuf  // gobuf arg to morestack
	divmod  uint32 // div/mod denominator for arm - known to liblink
	_       uint32 // align next field to 8 bytes

	// Fields not known to debuggers.
	procid        uint64            // for debuggers, but offset not hard-coded
	gsignal       *g                // signal-handling g
	goSigStack    gsignalStack      // Go-allocated signal handling stack
	sigmask       sigset            // storage for saved signal mask
	tls           [tlsSlots]uintptr // thread-local storage (for x86 extern register)
	mstartfn      func()
	curg          *g       // current running goroutine
	caughtsig     guintptr // goroutine running during fatal signal
	p             puintptr // attached p for executing go code (nil if not executing go code)
	nextp         puintptr
	oldp          puintptr // the p that was attached before executing a syscall
	id            int64
	mallocing     int32
	throwing      throwType
	preemptoff    string // if != "", keep curg running on this m
	locks         int32
	dying         int32
	profilehz     int32
	spinning      bool // m is out of work and is actively looking for work
	blocked       bool // m is blocked on a note
	newSigstack   bool // minit on C thread called sigaltstack
	printlock     int8
	incgo         bool          // m is executing a cgo call
	isextra       bool          // m is an extra m
	isExtraInC    bool          // m is an extra m that is not executing Go code
	isExtraInSig  bool          // m is an extra m in a signal handler
	freeWait      atomic.Uint32 // Whether it is safe to free g0 and delete m (one of freeMRef, freeMStack, freeMWait)
	needextram    bool
	traceback     uint8
	ncgocall      uint64        // number of cgo calls in total
	ncgo          int32         // number of cgo calls currently in progress
	cgoCallersUse atomic.Uint32 // if non-zero, cgoCallers in use temporarily
	cgoCallers    *cgoCallers   // cgo traceback if crashing in cgo call
	park          note
	alllink       *m // on allm
	schedlink     muintptr
	lockedg       guintptr
	createstack   [32]uintptr // stack that created this thread, it's used for StackRecord.Stack0, so it must align with it.
	lockedExt     uint32      // tracking for external LockOSThread
	lockedInt     uint32      // tracking for internal lockOSThread
	nextwaitm     muintptr    // next m waiting for lock

	mLockProfile mLockProfile // fields relating to runtime.lock contention

	// wait* are used to carry arguments from gopark into park_m, because
	// there's no stack to put them on. That is their sole purpose.
	waitunlockf          func(*g, unsafe.Pointer) bool
	waitlock             unsafe.Pointer
	waitTraceBlockReason traceBlockReason
	waitTraceSkip        int

	syscalltick uint32
	freelink    *m // on sched.freem
	trace       mTraceState

	// these are here because they are too large to be on the stack
	// of low-level NOSPLIT functions.
	libcall   libcall
	libcallpc uintptr // for cpu profiler
	libcallsp uintptr
	libcallg  guintptr
	syscall   libcall // stores syscall parameters on windows

	vdsoSP uintptr // SP for traceback while in VDSO call (0 if not in call)
	vdsoPC uintptr // PC for traceback while in VDSO call

	// preemptGen counts the number of completed preemption
	// signals. This is used to detect when a preemption is
	// requested, but fails.
	preemptGen atomic.Uint32

	// Whether this is a pending preemption signal on this M.
	signalPending atomic.Uint32

	// pcvalue lookup cache
	pcvalueCache pcvalueCache

	dlogPerM

	mOS

	chacha8   chacha8rand.State
	cheaprand uint64

	// Up to 10 locks held by this m, maintained by the lock ranking code.
	locksHeldLen int
	locksHeld    [10]heldLockInfo
}

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type p struct {
	id          int32
	status      uint32 // one of pidle/prunning/...
	link        puintptr
	schedtick   uint32     // incremented on every scheduler call
	syscalltick uint32     // incremented on every system call
	sysmontick  sysmontick // last tick observed by sysmon
	m           muintptr   // back-link to associated m (nil if idle)
	mcache      *mcache
	pcache      pageCache
	raceprocctx uintptr

	deferpool    []*_defer // pool of available defer structs (see panic.go)
	deferpoolbuf [32]*_defer

	// Cache of goroutine ids, amortizes accesses to runtime·sched.goidgen.
	goidcache    uint64
	goidcacheend uint64

	// Queue of runnable goroutines. Accessed without lock.
	runqhead uint32
	runqtail uint32
	runq     [256]guintptr
	// runnext, if non-nil, is a runnable G that was ready'd by
	// the current G and should be run next instead of what's in
	// runq if there's time remaining in the running G's time
	// slice. It will inherit the time left in the current time
	// slice. If a set of goroutines is locked in a
	// communicate-and-wait pattern, this schedules that set as a
	// unit and eliminates the (potentially large) scheduling
	// latency that otherwise arises from adding the ready'd
	// goroutines to the end of the run queue.
	//
	// Note that while other P's may atomically CAS this to zero,
	// only the owner P can CAS it to a valid G.
	runnext guintptr

	// Available G's (status == Gdead)
	gFree struct {
		gList
		n int32
	}

	sudogcache []*sudog
	sudogbuf   [128]*sudog

	// Cache of mspan objects from the heap.
	mspancache struct {
		// We need an explicit length here because this field is used
		// in allocation codepaths where write barriers are not allowed,
		// and eliminating the write barrier/keeping it eliminated from
		// slice updates is tricky, more so than just managing the length
		// ourselves.
		len int
		buf [128]*mspan
	}

	// Cache of a single pinner object to reduce allocations from repeated
	// pinner creation.
	pinnerCache *pinner

	trace pTraceState

	palloc persistentAlloc // per-P to avoid mutex

	// The when field of the first entry on the timer heap.
	// This is 0 if the timer heap is empty.
	timer0When atomic.Int64

	// The earliest known nextwhen field of a timer with
	// timerModifiedEarlier status. Because the timer may have been
	// modified again, there need not be any timer with this value.
	// This is 0 if there are no timerModifiedEarlier timers.
	timerModifiedEarliest atomic.Int64

	// Per-P GC state
	gcAssistTime         int64 // Nanoseconds in assistAlloc
	gcFractionalMarkTime int64 // Nanoseconds in fractional mark worker (atomic)

	// limiterEvent tracks events for the GC CPU limiter.
	limiterEvent limiterEvent

	// gcMarkWorkerMode is the mode for the next mark worker to run in.
	// That is, this is used to communicate with the worker goroutine
	// selected for immediate execution by
	// gcController.findRunnableGCWorker. When scheduling other goroutines,
	// this field must be set to gcMarkWorkerNotWorker.
	gcMarkWorkerMode gcMarkWorkerMode
	// gcMarkWorkerStartTime is the nanotime() at which the most recent
	// mark worker started.
	gcMarkWorkerStartTime int64

	// gcw is this P's GC work buffer cache. The work buffer is
	// filled by write barriers, drained by mutator assists, and
	// disposed on certain GC state transitions.
	gcw gcWork

	// wbBuf is this P's GC write barrier buffer.
	//
	// TODO: Consider caching this in the running G.
	wbBuf wbBuf

	runSafePointFn uint32 // if 1, run sched.safePointFn at next safe point

	// statsSeq is a counter indicating whether this P is currently
	// writing any stats. Its value is even when not, odd when it is.
	statsSeq atomic.Uint32

	// Lock for timers. We normally access the timers while running
	// on this P, but the scheduler can also do it from a different P.
	timersLock mutex

	// Actions to take at some time. This is used to implement the
	// standard library's time package.
	// Must hold timersLock to access.
	timers []*timer

	// Number of timers in P's heap.
	numTimers atomic.Uint32

	// Number of timerDeleted timers in P's heap.
	deletedTimers atomic.Uint32

	// Race context used while executing timer functions.
	timerRaceCtx uintptr

	// maxStackScanDelta accumulates the amount of stack space held by
	// live goroutines (i.e. those eligible for stack scanning).
	// Flushed to gcController.maxStackScan once maxStackScanSlack
	// or -maxStackScanSlack is reached.
	maxStackScanDelta int64

	// gc-time statistics about current goroutines
	// Note that this differs from maxStackScan in that this
	// accumulates the actual stack observed to be used at GC time (hi - sp),
	// not an instantaneous measure of the total stack size that might need
	// to be scanned (hi - lo).
	scannedStackSize uint64 // stack size of goroutines scanned by this P
	scannedStacks    uint64 // number of goroutines scanned by this P

	// preempt is set to indicate that this P should be enter the
	// scheduler ASAP (regardless of what G is running on it).
	preempt bool

	// pageTraceBuf is a buffer for writing out page allocation/free/scavenge traces.
	//
	// Used only if GOEXPERIMENT=pagetrace.
	pageTraceBuf pageTraceBuf

	// Padding is no longer needed. False sharing is now not a worry because p is large enough
	// that its size class is an integer multiple of the cache line size (for any of our architectures).
}

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type gobuf struct {
	// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
	//
	// ctxt is unusual with respect to GC: it may be a
	// heap-allocated funcval, so GC needs to track it, but it
	// needs to be set and cleared from assembly, where it's
	// difficult to have write barriers. However, ctxt is really a
	// saved, live register, and we only ever exchange it between
	// the real register and the gobuf. Hence, we treat it as a
	// root during stack scanning, which means assembly that saves
	// and restores it doesn't need write barriers. It's still
	// typed as a pointer so that any other writes from Go get
	// write barriers.
	sp   uintptr
	pc   uintptr
	g    guintptr
	ctxt unsafe.Pointer
	ret  uintptr
	lr   uintptr
	bp   uintptr // for framepointer-enabled architectures
}

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package main

import (
	"fmt"
	"time"
)

func main() {
	fmt.Println("Before: ", time.Now())

	time.Sleep(30 * time.Minute)

	fmt.Println("After: ", time.Now())
}

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time.Sleep -> 
  runtime.timeSleep { 
    ...
    gp := getg()
    t := gp.timer
    ...
    t.arg = gp
    ...
  } ->
    gopark(resetForSleep, unsafe.Pointer(t), waitReasonSleep, traceBlockSleep, 1)

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package main

import (
	"fmt"
	"sync"
	"time"
)

func main() {
	var ch = make(chan int)
	var wg = sync.WaitGroup{}
	wg.Add(2)

	go func(ch chan<- int) {
		defer close(ch)
		defer wg.Done()

		time.Sleep(time.Second)
		ch <- 1
	}(ch)

	go func(ch <-chan int) {
		defer wg.Done()
		val := <-ch
		fmt.Println(val)
	}(ch)

	wg.Wait()
}

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ch<- ->
  runtime.chansend1 -> 
  runtime.chansend {   
    ...
    gp := getg()
    mysg := acquireSudog()
    ...
    gp.waiting = mysg
    gp.param = nil
    c.sendq.enqueue(mysg)
    // Signal to anyone trying to shrink our stack that we're about
    // to park on a channel. The window between when this G's status
    // changes and when we set gp.activeStackChans is not safe for
    // stack shrinking.
    gp.parkingOnChan.Store(true)
    gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanSend, traceBlockChanSend, 2)
    ...
  } ->
  gopark

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<-ch ->
  runtime.chanrecv1(c *hchan, elem unsafe.Pointer) ->
    runtime.chanrecv(c *hchan, ep unsafe.Pointer, block bool) (selected, received bool) {
      ...
      // no sender available: block on this channel.
      gp := getg()
      mysg := acquireSudog()
      ...
      gp.waiting = mysg
      mysg.g = gp
      ...
      c.recvq.enqueue(mysg)
      // Signal to anyone trying to shrink our stack that we're about
      // to park on a channel. The window between when this G's status
      // changes and when we set gp.activeStackChans is not safe for
      // stack shrinking.
      gp.parkingOnChan.Store(true)
      gopark(chanparkcommit, unsafe.Pointer(&c.lock), waitReasonChanReceive, traceBlockChanRecv, 2)
    } ->
      runtime.gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int)

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// sudog (pseudo-g) represents a g in a wait list, such as for sending/receiving
// on a channel.
//
// sudog is necessary because the g ↔ synchronization object relation
// is many-to-many. A g can be on many wait lists, so there may be
// many sudogs for one g; and many gs may be waiting on the same
// synchronization object, so there may be many sudogs for one object.
//
// sudogs are allocated from a special pool. Use acquireSudog and
// releaseSudog 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

	// waiters is a count of semaRoot waiting list other than head of list,
	// clamped to a uint16 to fit in unused space.
	// Only meaningful at the head of the list.
	// (If we wanted to be overly clever, we could store a high 16 bits
	// in the second entry in the list.)
	waiters uint16

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

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type hchan struct {
	qcount   uint           // total data in the queue
	dataqsiz uint           // size of the circular queue
	buf      unsafe.Pointer // points to an array of dataqsiz elements
	elemsize uint16
	closed   uint32
	elemtype *_type // element type
	sendx    uint   // send index
	recvx    uint   // receive index
	recvq    waitq  // list of recv waiters
	sendq    waitq  // list of send waiters

	// lock protects all fields in hchan, as well as several
	// fields in sudogs blocked on this channel.
	//
	// Do not change another G's status while holding this lock
	// (in particular, do not ready a G), as this can deadlock
	// with stack shrinking.
	lock mutex
}

type waitq struct {
	first *sudog
	last  *sudog
}

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package main

import (
	"net"
)

func main() {
	l, _ := net.Listen("tcp", ":6633")

	for {
		conn, _ := l.Accept()

		go func() {
			defer conn.Close()

			var buf = make([]byte, 512)
			n, _ := conn.Read(buf)
			n, _ = conn.Write(buf)
		}()
	}
}

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conn.Read(buf) --->
  net.(*conn).Read(b []byte) (int, error) --->
    net.(*netFD).Read(p []byte) (n int, err error) --->
      poll.(*FD).Read(p []byte) (int, error) --->
        poll.(*pollDesc).waitRead(isFile bool) error --->
          poll.(*pollDesc).wait(mode int, isFile bool) error --->
            runtime.poll_runtime_pollWait(pd *pollDesc, mode int) int --->
              runtime.netpollblock(pd *pollDesc, mode int32, waitio bool) bool {
                gpp := &pd.rg
                if mode == 'w' {
                  gpp = &pd.wg
                }
                ...
                gopark(netpollblockcommit, unsafe.Pointer(gpp), waitReasonIOWait, traceBlockNet, 5)
                ...
              } --->
                gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int)

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conn.Write(buf) --->
  net.(*conn).Write(b []byte) (int, error) --->
    net.(*netFD).Write(p []byte) (n int, err error) --->
      poll.(*FD).Write(p []byte) (int, error) --->
        poll.(*pollDesc).waitWrite(isFile bool) error --->
          poll.(*pollDesc).wait(mode int, isFile bool) error --->
            runtime.poll_runtime_pollWait(pd *pollDesc, mode int) int --->
              runtime.netpollblock(pd *pollDesc, mode int32, waitio bool) bool {
                gpp := &pd.rg
                if mode == 'w' {
                  gpp = &pd.wg
                }
                ...
                gopark(netpollblockcommit, unsafe.Pointer(gpp), waitReasonIOWait, traceBlockNet, 5)
                ...
              } --->
                gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int)

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package main

import (
	"fmt"
	"sync"
	"time"
)

var mtx sync.Mutex

func main() {
	go func() {
		mtx.Lock()
		defer mtx.Unlock()

		fmt.Printf("Start\n")
		time.Sleep(time.Second * 10)
		fmt.Printf("End\n")
	}()

	time.Sleep(time.Second) // Ensure child goroutine gets the mutex before main goroutine

	fmt.Printf("Try to acquire mutex\n")
	mtx.Lock()
	fmt.Printf("Main goroutine\n")
	mtx.Unlock()
}

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mtx.Lock() --->
	sync.(*Mutex).Lock() --->
		sync.(*Mutex) lockSlow() --->
			sync.runtime_SemacquireMutex(s *uint32, lifo bool, skipframes int) --->
				sync.sync_runtime_SemacquireMutex(addr *uint32, lifo bool, skipframes int) --->
					runtime.semacquire1(addr *uint32, lifo bool, profile semaProfileFlags, skipframes int, reason waitReason) {
						gp := getg()
						if gp != gp.m.curg {
							throw("semacquire not on the G stack")
						}

						// Easy case.
						if cansemacquire(addr) {
							return
						}

						// Harder case:
						//	increment waiter count
						//	try cansemacquire one more time, return if succeeded
						//	enqueue itself as a waiter
						//	sleep
						//	(waiter descriptor is dequeued by signaler)
						s := acquireSudog()
						root := semtable.rootFor(addr)
						...	
							// Any semrelease after the cansemacquire knows we're waiting
							// (we set nwait above), so go to sleep.
							root.queue(addr, s, lifo)
							goparkunlock(&root.lock, reason, traceBlockSync, 4+skipframes)
							...
					} --->
						goparkunlock(lock *mutex, reason waitReason, traceReason traceBlockReason, traceskip int) --->
							gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int)	

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// Asynchronous semaphore for sync.Mutex.

// A semaRoot holds a balanced tree of sudog with distinct addresses (s.elem).
// Each of those sudog may in turn point (through s.waitlink) to a list
// of other sudogs waiting on the same address.
// The operations on the inner lists of sudogs with the same address
// are all O(1). The scanning of the top-level semaRoot list is O(log n),
// where n is the number of distinct addresses with goroutines blocked
// on them that hash to the given semaRoot.
// See golang.org/issue/17953 for a program that worked badly
// before we introduced the second level of list, and
// BenchmarkSemTable/OneAddrCollision/* for a benchmark that exercises this.
type semaRoot struct {
	lock  mutex
	treap *sudog        // root of balanced tree of unique waiters.
	nwait atomic.Uint32 // Number of waiters. Read w/o the lock.
}

// Prime to not correlate with any user patterns.
const semTabSize = 251

type semTable [semTabSize]struct {
	root semaRoot
	pad  [cpu.CacheLinePadSize - unsafe.Sizeof(semaRoot{})]byte
}