1 files changed, 231 insertions, 0 deletions
diff --git a/src/sync/rwmutex.go b/src/sync/rwmutex.go
new file mode 100644
index 0000000..ad52951
--- /dev/null
+++ b/src/sync/rwmutex.go
@@ -0,0 +1,231 @@
+// Copyright 2009 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 sync
+
+import (
+	"internal/race"
+	"sync/atomic"
+	"unsafe"
+)
+
+// There is a modified copy of this file in runtime/rwmutex.go.
+// If you make any changes here, see if you should make them there.
+
+// A RWMutex is a reader/writer mutual exclusion lock.
+// The lock can be held by an arbitrary number of readers or a single writer.
+// The zero value for a RWMutex is an unlocked mutex.
+//
+// A RWMutex must not be copied after first use.
+//
+// If a goroutine holds a RWMutex for reading and another goroutine might
+// call Lock, no goroutine should expect to be able to acquire a read lock
+// until the initial read lock is released. In particular, this prohibits
+// recursive read locking. This is to ensure that the lock eventually becomes
+// available; a blocked Lock call excludes new readers from acquiring the
+// lock.
+//
+// In the terminology of the Go memory model,
+// the n'th call to Unlock “synchronizes before” the m'th call to Lock
+// for any n < m, just as for Mutex.
+// For any call to RLock, there exists an n such that
+// the n'th call to Unlock “synchronizes before” that call to RLock,
+// and the corresponding call to RUnlock “synchronizes before”
+// the n+1'th call to Lock.
+type RWMutex struct {
+	w           Mutex        // held if there are pending writers
+	writerSem   uint32       // semaphore for writers to wait for completing readers
+	readerSem   uint32       // semaphore for readers to wait for completing writers
+	readerCount atomic.Int32 // number of pending readers
+	readerWait  atomic.Int32 // number of departing readers
+}
+
+const rwmutexMaxReaders = 1 << 30
+
+// Happens-before relationships are indicated to the race detector via:
+// - Unlock  -> Lock:  readerSem
+// - Unlock  -> RLock: readerSem
+// - RUnlock -> Lock:  writerSem
+//
+// The methods below temporarily disable handling of race synchronization
+// events in order to provide the more precise model above to the race
+// detector.
+//
+// For example, atomic.AddInt32 in RLock should not appear to provide
+// acquire-release semantics, which would incorrectly synchronize racing
+// readers, thus potentially missing races.
+
+// RLock locks rw for reading.
+//
+// It should not be used for recursive read locking; a blocked Lock
+// call excludes new readers from acquiring the lock. See the
+// documentation on the RWMutex type.
+func (rw *RWMutex) RLock() {
+	if race.Enabled {
+		_ = rw.w.state
+		race.Disable()
+	}
+	if rw.readerCount.Add(1) < 0 {
+		// A writer is pending, wait for it.
+		runtime_SemacquireRWMutexR(&rw.readerSem, false, 0)
+	}
+	if race.Enabled {
+		race.Enable()
+		race.Acquire(unsafe.Pointer(&rw.readerSem))
+	}
+}
+
+// TryRLock tries to lock rw for reading and reports whether it succeeded.
+//
+// Note that while correct uses of TryRLock do exist, they are rare,
+// and use of TryRLock is often a sign of a deeper problem
+// in a particular use of mutexes.
+func (rw *RWMutex) TryRLock() bool {
+	if race.Enabled {
+		_ = rw.w.state
+		race.Disable()
+	}
+	for {
+		c := rw.readerCount.Load()
+		if c < 0 {
+			if race.Enabled {
+				race.Enable()
+			}
+			return false
+		}
+		if rw.readerCount.CompareAndSwap(c, c+1) {
+			if race.Enabled {
+				race.Enable()
+				race.Acquire(unsafe.Pointer(&rw.readerSem))
+			}
+			return true
+		}
+	}
+}
+
+// RUnlock undoes a single RLock call;
+// it does not affect other simultaneous readers.
+// It is a run-time error if rw is not locked for reading
+// on entry to RUnlock.
+func (rw *RWMutex) RUnlock() {
+	if race.Enabled {
+		_ = rw.w.state
+		race.ReleaseMerge(unsafe.Pointer(&rw.writerSem))
+		race.Disable()
+	}
+	if r := rw.readerCount.Add(-1); r < 0 {
+		// Outlined slow-path to allow the fast-path to be inlined
+		rw.rUnlockSlow(r)
+	}
+	if race.Enabled {
+		race.Enable()
+	}
+}
+
+func (rw *RWMutex) rUnlockSlow(r int32) {
+	if r+1 == 0 || r+1 == -rwmutexMaxReaders {
+		race.Enable()
+		fatal("sync: RUnlock of unlocked RWMutex")
+	}
+	// A writer is pending.
+	if rw.readerWait.Add(-1) == 0 {
+		// The last reader unblocks the writer.
+		runtime_Semrelease(&rw.writerSem, false, 1)
+	}
+}
+
+// Lock locks rw for writing.
+// If the lock is already locked for reading or writing,
+// Lock blocks until the lock is available.
+func (rw *RWMutex) Lock() {
+	if race.Enabled {
+		_ = rw.w.state
+		race.Disable()
+	}
+	// First, resolve competition with other writers.
+	rw.w.Lock()
+	// Announce to readers there is a pending writer.
+	r := rw.readerCount.Add(-rwmutexMaxReaders) + rwmutexMaxReaders
+	// Wait for active readers.
+	if r != 0 && rw.readerWait.Add(r) != 0 {
+		runtime_SemacquireRWMutex(&rw.writerSem, false, 0)
+	}
+	if race.Enabled {
+		race.Enable()
+		race.Acquire(unsafe.Pointer(&rw.readerSem))
+		race.Acquire(unsafe.Pointer(&rw.writerSem))
+	}
+}
+
+// TryLock tries to lock rw for writing and reports whether it succeeded.
+//
+// Note that while correct uses of TryLock do exist, they are rare,
+// and use of TryLock is often a sign of a deeper problem
+// in a particular use of mutexes.
+func (rw *RWMutex) TryLock() bool {
+	if race.Enabled {
+		_ = rw.w.state
+		race.Disable()
+	}
+	if !rw.w.TryLock() {
+		if race.Enabled {
+			race.Enable()
+		}
+		return false
+	}
+	if !rw.readerCount.CompareAndSwap(0, -rwmutexMaxReaders) {
+		rw.w.Unlock()
+		if race.Enabled {
+			race.Enable()
+		}
+		return false
+	}
+	if race.Enabled {
+		race.Enable()
+		race.Acquire(unsafe.Pointer(&rw.readerSem))
+		race.Acquire(unsafe.Pointer(&rw.writerSem))
+	}
+	return true
+}
+
+// Unlock unlocks rw for writing. It is a run-time error if rw is
+// not locked for writing on entry to Unlock.
+//
+// As with Mutexes, a locked RWMutex is not associated with a particular
+// goroutine. One goroutine may RLock (Lock) a RWMutex and then
+// arrange for another goroutine to RUnlock (Unlock) it.
+func (rw *RWMutex) Unlock() {
+	if race.Enabled {
+		_ = rw.w.state
+		race.Release(unsafe.Pointer(&rw.readerSem))
+		race.Disable()
+	}
+
+	// Announce to readers there is no active writer.
+	r := rw.readerCount.Add(rwmutexMaxReaders)
+	if r >= rwmutexMaxReaders {
+		race.Enable()
+		fatal("sync: Unlock of unlocked RWMutex")
+	}
+	// Unblock blocked readers, if any.
+	for i := 0; i < int(r); i++ {
+		runtime_Semrelease(&rw.readerSem, false, 0)
+	}
+	// Allow other writers to proceed.
+	rw.w.Unlock()
+	if race.Enabled {
+		race.Enable()
+	}
+}
+
+// RLocker returns a Locker interface that implements
+// the Lock and Unlock methods by calling rw.RLock and rw.RUnlock.
+func (rw *RWMutex) RLocker() Locker {
+	return (*rlocker)(rw)
+}
+
+type rlocker RWMutex
+
+func (r *rlocker) Lock()   { (*RWMutex)(r).RLock() }
+func (r *rlocker) Unlock() { (*RWMutex)(r).RUnlock() }