1 files changed, 167 insertions, 0 deletions
diff --git a/src/runtime/rwmutex.go b/src/runtime/rwmutex.go
new file mode 100644
index 0000000..34d8f67
--- /dev/null
+++ b/src/runtime/rwmutex.go
@@ -0,0 +1,167 @@
+// 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)
+}