// 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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"runtime/internal/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 uint32 // number of pending readers
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readerWait uint32 // number of departing readers
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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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acquirem()
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if int32(atomic.Xadd(&rw.readerCount, 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 := int32(atomic.Xadd(&rw.readerCount, -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 atomic.Xadd(&rw.readerWait, -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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releasem(getg().m)
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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 := int32(atomic.Xadd(&rw.readerCount, -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 && atomic.Xadd(&rw.readerWait, 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 := int32(atomic.Xadd(&rw.readerCount, 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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