mirror of https://go.googlesource.com/go
165 lines
5.0 KiB
Go
165 lines
5.0 KiB
Go
// Copyright 2017 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package runtime
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import (
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"internal/runtime/atomic"
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)
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// This is a copy of sync/rwmutex.go rewritten to work in the runtime.
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// A rwmutex is a reader/writer mutual exclusion lock.
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// The lock can be held by an arbitrary number of readers or a single writer.
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// This is a variant of sync.RWMutex, for the runtime package.
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// Like mutex, rwmutex blocks the calling M.
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// It does not interact with the goroutine scheduler.
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type rwmutex struct {
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rLock mutex // protects readers, readerPass, writer
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readers muintptr // list of pending readers
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readerPass uint32 // number of pending readers to skip readers list
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wLock mutex // serializes writers
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writer muintptr // pending writer waiting for completing readers
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readerCount atomic.Int32 // number of pending readers
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readerWait atomic.Int32 // number of departing readers
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readRank lockRank // semantic lock rank for read locking
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}
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// Lock ranking an rwmutex has two aspects:
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//
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// Semantic ranking: this rwmutex represents some higher level lock that
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// protects some resource (e.g., allocmLock protects creation of new Ms). The
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// read and write locks of that resource need to be represented in the lock
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// rank.
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//
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// Internal ranking: as an implementation detail, rwmutex uses two mutexes:
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// rLock and wLock. These have lock order requirements: wLock must be locked
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// before rLock. This also needs to be represented in the lock rank.
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//
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// Semantic ranking is represented by acquiring readRank during read lock and
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// writeRank during write lock.
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//
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// wLock is held for the duration of a write lock, so it uses writeRank
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// directly, both for semantic and internal ranking. rLock is only held
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// temporarily inside the rlock/lock methods, so it uses readRankInternal to
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// represent internal ranking. Semantic ranking is represented by a separate
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// acquire of readRank for the duration of a read lock.
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//
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// The lock ranking must document this ordering:
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// - readRankInternal is a leaf lock.
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// - readRank is taken before readRankInternal.
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// - writeRank is taken before readRankInternal.
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// - readRank is placed in the lock order wherever a read lock of this rwmutex
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// belongs.
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// - writeRank is placed in the lock order wherever a write lock of this
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// rwmutex belongs.
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func (rw *rwmutex) init(readRank, readRankInternal, writeRank lockRank) {
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rw.readRank = readRank
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lockInit(&rw.rLock, readRankInternal)
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lockInit(&rw.wLock, writeRank)
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}
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const rwmutexMaxReaders = 1 << 30
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// rlock locks rw for reading.
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func (rw *rwmutex) rlock() {
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// The reader must not be allowed to lose its P or else other
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// things blocking on the lock may consume all of the Ps and
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// deadlock (issue #20903). Alternatively, we could drop the P
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// while sleeping.
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acquireLockRankAndM(rw.readRank)
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lockWithRankMayAcquire(&rw.rLock, getLockRank(&rw.rLock))
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if rw.readerCount.Add(1) < 0 {
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// A writer is pending. Park on the reader queue.
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systemstack(func() {
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lock(&rw.rLock)
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if rw.readerPass > 0 {
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// Writer finished.
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rw.readerPass -= 1
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unlock(&rw.rLock)
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} else {
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// Queue this reader to be woken by
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// the writer.
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m := getg().m
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m.schedlink = rw.readers
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rw.readers.set(m)
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unlock(&rw.rLock)
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notesleep(&m.park)
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noteclear(&m.park)
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}
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})
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}
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}
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// runlock undoes a single rlock call on rw.
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func (rw *rwmutex) runlock() {
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if r := rw.readerCount.Add(-1); r < 0 {
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if r+1 == 0 || r+1 == -rwmutexMaxReaders {
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throw("runlock of unlocked rwmutex")
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}
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// A writer is pending.
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if rw.readerWait.Add(-1) == 0 {
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// The last reader unblocks the writer.
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lock(&rw.rLock)
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w := rw.writer.ptr()
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if w != nil {
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notewakeup(&w.park)
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}
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unlock(&rw.rLock)
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}
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}
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releaseLockRankAndM(rw.readRank)
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}
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// lock locks rw for writing.
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func (rw *rwmutex) lock() {
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// Resolve competition with other writers and stick to our P.
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lock(&rw.wLock)
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m := getg().m
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// Announce that there is a pending writer.
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r := rw.readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders
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// Wait for any active readers to complete.
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lock(&rw.rLock)
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if r != 0 && rw.readerWait.Add(r) != 0 {
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// Wait for reader to wake us up.
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systemstack(func() {
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rw.writer.set(m)
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unlock(&rw.rLock)
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notesleep(&m.park)
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noteclear(&m.park)
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})
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} else {
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unlock(&rw.rLock)
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}
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}
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// unlock unlocks rw for writing.
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func (rw *rwmutex) unlock() {
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// Announce to readers that there is no active writer.
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r := rw.readerCount.Add(rwmutexMaxReaders)
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if r >= rwmutexMaxReaders {
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throw("unlock of unlocked rwmutex")
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}
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// Unblock blocked readers.
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lock(&rw.rLock)
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for rw.readers.ptr() != nil {
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reader := rw.readers.ptr()
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rw.readers = reader.schedlink
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reader.schedlink.set(nil)
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notewakeup(&reader.park)
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r -= 1
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}
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// If r > 0, there are pending readers that aren't on the
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// queue. Tell them to skip waiting.
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rw.readerPass += uint32(r)
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unlock(&rw.rLock)
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// Allow other writers to proceed.
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unlock(&rw.wLock)
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}
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