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