pkg/sync/gate_unsafe.go

// Copyright 2018 The gVisor Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package sync

import (
	"fmt"
	"math"
	"sync/atomic"
	"unsafe"

	"gvisor.dev/gvisor/pkg/gohacks"
)

// Gate is a synchronization primitive that allows concurrent goroutines to
// "enter" it as long as it hasn't been closed yet. Once it's been closed,
// goroutines cannot enter it anymore, but are allowed to leave, and the closer
// will be informed when all goroutines have left.
//
// Gate is similar to WaitGroup:
//
// - Gate.Enter() is analogous to WaitGroup.Add(1), but may be called even if
// the Gate counter is 0 and fails if Gate.Close() has been called.
//
// - Gate.Leave() is equivalent to WaitGroup.Done().
//
// - Gate.Close() is analogous to WaitGroup.Wait(), but also causes future
// calls to Gate.Enter() to fail and may only be called once, from a single
// goroutine.
//
// This is useful, for example, in cases when a goroutine is trying to clean up
// an object for which multiple goroutines have pointers. In such a case, users
// would be required to enter and leave the Gate, and the cleaner would wait
// until all users are gone (and no new ones are allowed) before proceeding.
//
// Users:
//
//	if !g.Enter() {
//		// Gate is closed, we can't use the object.
//		return
//	}
//
//	// Do something with object.
//	[...]
//
//	g.Leave()
//
// Closer:
//
//	// Prevent new users from using the object, and wait for the existing
//	// ones to complete.
//	g.Close()
//
//	// Clean up the object.
//	[...]
//
type Gate struct {
	userCount int32
	closingG  uintptr
}

const preparingG = 1

// Enter tries to enter the gate. It will succeed if it hasn't been closed yet,
// in which case the caller must eventually call Leave().
//
// This function is thread-safe.
func (g *Gate) Enter() bool {
	if atomic.AddInt32(&g.userCount, 1) > 0 {
		return true
	}
	g.leaveAfterFailedEnter()
	return false
}

// leaveAfterFailedEnter is identical to Leave, but is marked noinline to
// prevent it from being inlined into Enter, since as of this writing inlining
// Leave into Enter prevents Enter from being inlined into its callers.
//go:noinline
func (g *Gate) leaveAfterFailedEnter() {
	if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 {
		g.leaveClosed()
	}
}

// Leave leaves the gate. This must only be called after a successful call to
// Enter(). If the gate has been closed and this is the last one inside the
// gate, it will notify the closer that the gate is done.
//
// This function is thread-safe.
func (g *Gate) Leave() {
	if atomic.AddInt32(&g.userCount, -1) == math.MinInt32 {
		g.leaveClosed()
	}
}

func (g *Gate) leaveClosed() {
	if atomic.LoadUintptr(&g.closingG) == 0 {
		return
	}
	if g := atomic.SwapUintptr(&g.closingG, 0); g > preparingG {
		goready(g, 0)
	}
}

// Close closes the gate, causing future calls to Enter to fail, and waits
// until all goroutines that are currently inside the gate leave before
// returning.
//
// Only one goroutine can call this function.
func (g *Gate) Close() {
	if atomic.LoadInt32(&g.userCount) == math.MinInt32 {
		// The gate is already closed, with no goroutines inside. For legacy
		// reasons, we have to allow Close to be called again in this case.
		return
	}
	if v := atomic.AddInt32(&g.userCount, math.MinInt32); v == math.MinInt32 {
		// userCount was already 0.
		return
	} else if v >= 0 {
		panic("concurrent Close of sync.Gate")
	}

	if g := atomic.SwapUintptr(&g.closingG, preparingG); g != 0 {
		panic(fmt.Sprintf("invalid sync.Gate.closingG during Close: %#x", g))
	}
	if atomic.LoadInt32(&g.userCount) == math.MinInt32 {
		// The last call to Leave arrived while we were setting up closingG.
		return
	}
	// WaitReasonSemacquire/TraceEvGoBlockSync are consistent with WaitGroup.
	gopark(gateCommit, gohacks.Noescape(unsafe.Pointer(&g.closingG)), WaitReasonSemacquire, TraceEvGoBlockSync, 0)
}

//go:norace
//go:nosplit
func gateCommit(g uintptr, closingG unsafe.Pointer) bool {
	return RaceUncheckedAtomicCompareAndSwapUintptr((*uintptr)(closingG), preparingG, g)
}