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// Copyright 2013 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
package poll
import "sync/atomic"
// fdMutex is a specialized synchronization primitive that manages
// lifetime of an fd and serializes access to Read, Write and Close
// methods on FD.
type fdMutex struct {
state uint64
rsema uint32
wsema uint32
}
// fdMutex.state is organized as follows:
// 1 bit - whether FD is closed, if set all subsequent lock operations will fail.
// 1 bit - lock for read operations.
// 1 bit - lock for write operations.
// 20 bits - total number of references (read+write+misc).
// 20 bits - number of outstanding read waiters.
// 20 bits - number of outstanding write waiters.
const (
mutexClosed = 1 << 0
mutexRLock = 1 << 1
mutexWLock = 1 << 2
mutexRef = 1 << 3
mutexRefMask = (1<<20 - 1) << 3
mutexRWait = 1 << 23
mutexRMask = (1<<20 - 1) << 23
mutexWWait = 1 << 43
mutexWMask = (1<<20 - 1) << 43
)
const overflowMsg = "too many concurrent operations on a single file or socket (max 1048575)"
// Read operations must do rwlock(true)/rwunlock(true).
//
// Write operations must do rwlock(false)/rwunlock(false).
//
// Misc operations must do incref/decref.
// Misc operations include functions like setsockopt and setDeadline.
// They need to use incref/decref to ensure that they operate on the
// correct fd in presence of a concurrent close call (otherwise fd can
// be closed under their feet).
//
// Close operations must do increfAndClose/decref.
// incref adds a reference to mu.
// It reports whether mu is available for reading or writing.
func (mu *fdMutex) incref() bool {
for {
old := atomic.LoadUint64(&mu.state)
if old&mutexClosed != 0 {
return false
}
new := old + mutexRef
if new&mutexRefMask == 0 {
panic(overflowMsg)
}
if atomic.CompareAndSwapUint64(&mu.state, old, new) {
return true
}
}
}
// increfAndClose sets the state of mu to closed.
// It returns false if the file was already closed.
func (mu *fdMutex) increfAndClose() bool {
for {
old := atomic.LoadUint64(&mu.state)
if old&mutexClosed != 0 {
return false
}
// Mark as closed and acquire a reference.
new := (old | mutexClosed) + mutexRef
if new&mutexRefMask == 0 {
panic(overflowMsg)
}
// Remove all read and write waiters.
new &^= mutexRMask | mutexWMask
if atomic.CompareAndSwapUint64(&mu.state, old, new) {
// Wake all read and write waiters,
// they will observe closed flag after wakeup.
for old&mutexRMask != 0 {
old -= mutexRWait
runtime_Semrelease(&mu.rsema)
}
for old&mutexWMask != 0 {
old -= mutexWWait
runtime_Semrelease(&mu.wsema)
}
return true
}
}
}
// decref removes a reference from mu.
// It reports whether there is no remaining reference.
func (mu *fdMutex) decref() bool {
for {
old := atomic.LoadUint64(&mu.state)
if old&mutexRefMask == 0 {
panic("inconsistent poll.fdMutex")
}
new := old - mutexRef
if atomic.CompareAndSwapUint64(&mu.state, old, new) {
return new&(mutexClosed|mutexRefMask) == mutexClosed
}
}
}
// lock adds a reference to mu and locks mu.
// It reports whether mu is available for reading or writing.
func (mu *fdMutex) rwlock(read bool) bool {
var mutexBit, mutexWait, mutexMask uint64
var mutexSema *uint32
if read {
mutexBit = mutexRLock
mutexWait = mutexRWait
mutexMask = mutexRMask
mutexSema = &mu.rsema
} else {
mutexBit = mutexWLock
mutexWait = mutexWWait
mutexMask = mutexWMask
mutexSema = &mu.wsema
}
for {
old := atomic.LoadUint64(&mu.state)
if old&mutexClosed != 0 {
return false
}
var new uint64
if old&mutexBit == 0 {
// Lock is free, acquire it.
new = (old | mutexBit) + mutexRef
if new&mutexRefMask == 0 {
panic(overflowMsg)
}
} else {
// Wait for lock.
new = old + mutexWait
if new&mutexMask == 0 {
panic(overflowMsg)
}
}
if atomic.CompareAndSwapUint64(&mu.state, old, new) {
if old&mutexBit == 0 {
return true
}
runtime_Semacquire(mutexSema)
// The signaller has subtracted mutexWait.
}
}
}
// unlock removes a reference from mu and unlocks mu.
// It reports whether there is no remaining reference.
func (mu *fdMutex) rwunlock(read bool) bool {
var mutexBit, mutexWait, mutexMask uint64
var mutexSema *uint32
if read {
mutexBit = mutexRLock
mutexWait = mutexRWait
mutexMask = mutexRMask
mutexSema = &mu.rsema
} else {
mutexBit = mutexWLock
mutexWait = mutexWWait
mutexMask = mutexWMask
mutexSema = &mu.wsema
}
for {
old := atomic.LoadUint64(&mu.state)
if old&mutexBit == 0 || old&mutexRefMask == 0 {
panic("inconsistent poll.fdMutex")
}
// Drop lock, drop reference and wake read waiter if present.
new := (old &^ mutexBit) - mutexRef
if old&mutexMask != 0 {
new -= mutexWait
}
if atomic.CompareAndSwapUint64(&mu.state, old, new) {
if old&mutexMask != 0 {
runtime_Semrelease(mutexSema)
}
return new&(mutexClosed|mutexRefMask) == mutexClosed
}
}
}
// Implemented in runtime package.
func runtime_Semacquire(sema *uint32)
func runtime_Semrelease(sema *uint32)
// incref adds a reference to fd.
// It returns an error when fd cannot be used.
func (fd *FD) incref() error {
if !fd.fdmu.incref() {
return errClosing(fd.isFile)
}
return nil
}
// decref removes a reference from fd.
// It also closes fd when the state of fd is set to closed and there
// is no remaining reference.
func (fd *FD) decref() error {
if fd.fdmu.decref() {
return fd.destroy()
}
return nil
}
// readLock adds a reference to fd and locks fd for reading.
// It returns an error when fd cannot be used for reading.
func (fd *FD) readLock() error {
if !fd.fdmu.rwlock(true) {
return errClosing(fd.isFile)
}
return nil
}
// readUnlock removes a reference from fd and unlocks fd for reading.
// It also closes fd when the state of fd is set to closed and there
// is no remaining reference.
func (fd *FD) readUnlock() {
if fd.fdmu.rwunlock(true) {
fd.destroy()
}
}
// writeLock adds a reference to fd and locks fd for writing.
// It returns an error when fd cannot be used for writing.
func (fd *FD) writeLock() error {
if !fd.fdmu.rwlock(false) {
return errClosing(fd.isFile)
}
return nil
}
// writeUnlock removes a reference from fd and unlocks fd for writing.
// It also closes fd when the state of fd is set to closed and there
// is no remaining reference.
func (fd *FD) writeUnlock() {
if fd.fdmu.rwunlock(false) {
fd.destroy()
}
}