8023234: StampedLock serializes readers on writer unlock

Summary: Sync-up the fix from jsr166 CVS, signal more readers on writer unlock
Reviewed-by: martin, shade
Contributed-by: NDoug Lea <dl@cs.oswego.edu>

src/share/classes/java/util/concurrent/locks/StampedLock.java

@@ -226,7 +226,11 @@ public class StampedLock implements java.io.Serializable {
      * incoming reader arrives while read lock is held but there is a
      * queued writer, this incoming reader is queued.  (This rule is
      * responsible for some of the complexity of method acquireRead,
-     * but without it, the lock becomes highly unfair.)
+     * but without it, the lock becomes highly unfair.) Method release
+     * does not (and sometimes cannot) itself wake up cowaiters. This
+     * is done by the primary thread, but helped by any other threads
+     * with nothing better to do in methods acquireRead and
+     * acquireWrite.
      *
      * These rules apply to threads actually queued. All tryLock forms
      * opportunistically try to acquire locks regardless of preference
@@ -267,11 +271,14 @@ public class StampedLock implements java.io.Serializable {
     /** Number of processors, for spin control */
     private static final int NCPU = Runtime.getRuntime().availableProcessors();
 
-    /** Maximum number of retries before blocking on acquisition */
+    /** Maximum number of retries before enqueuing on acquisition */
     private static final int SPINS = (NCPU > 1) ? 1 << 6 : 0;
 
+    /** Maximum number of retries before blocking at head on acquisition */
+    private static final int HEAD_SPINS = (NCPU > 1) ? 1 << 10 : 0;
+
     /** Maximum number of retries before re-blocking */
-    private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 12 : 0;
+    private static final int MAX_HEAD_SPINS = (NCPU > 1) ? 1 << 16 : 0;
 
     /** The period for yielding when waiting for overflow spinlock */
     private static final int OVERFLOW_YIELD_RATE = 7; // must be power 2 - 1
@@ -415,8 +422,8 @@ public class StampedLock implements java.io.Serializable {
      * @return a stamp that can be used to unlock or convert mode
      */
     public long readLock() {
-        long s, next;  // bypass acquireRead on fully unlocked case only
-        return ((((s = state) & ABITS) == 0L &&
+        long s = state, next;  // bypass acquireRead on common uncontended case
+        return ((whead == wtail && (s & ABITS) < RFULL &&
                  U.compareAndSwapLong(this, STATE, s, next = s + RUNIT)) ?
                 next : acquireRead(false, 0L));
     }
@@ -1012,18 +1019,9 @@ public class StampedLock implements java.io.Serializable {
                     if (t.status <= 0)
                         q = t;
             }
-            if (q != null) {
-                for (WNode r = q;;) {  // release co-waiters too
-                    if ((w = r.thread) != null) {
-                        r.thread = null;
+            if (q != null && (w = q.thread) != null)
                 U.unpark(w);
         }
-                    if ((r = q.cowait) == null)
-                        break;
-                    U.compareAndSwapObject(q, WCOWAIT, r, r.cowait);
-                }
-            }
-        }
     }
 
     /**
@@ -1038,22 +1036,22 @@ public class StampedLock implements java.io.Serializable {
     private long acquireWrite(boolean interruptible, long deadline) {
         WNode node = null, p;
         for (int spins = -1;;) { // spin while enqueuing
-            long s, ns;
-            if (((s = state) & ABITS) == 0L) {
+            long m, s, ns;
+            if ((m = (s = state) & ABITS) == 0L) {
                 if (U.compareAndSwapLong(this, STATE, s, ns = s + WBIT))
                     return ns;
             }
+            else if (spins < 0)
+                spins = (m == WBIT && wtail == whead) ? SPINS : 0;
             else if (spins > 0) {
                 if (LockSupport.nextSecondarySeed() >= 0)
                     --spins;
             }
             else if ((p = wtail) == null) { // initialize queue
-                WNode h = new WNode(WMODE, null);
-                if (U.compareAndSwapObject(this, WHEAD, null, h))
-                    wtail = h;
+                WNode hd = new WNode(WMODE, null);
+                if (U.compareAndSwapObject(this, WHEAD, null, hd))
+                    wtail = hd;
             }
-            else if (spins < 0)
-                spins = (p == whead) ? SPINS : 0;
             else if (node == null)
                 node = new WNode(WMODE, p);
             else if (node.prev != p)
@@ -1064,14 +1062,18 @@ public class StampedLock implements java.io.Serializable {
             }
         }
 
-        for (int spins = SPINS;;) {
-            WNode np, pp; int ps; long s, ns; Thread w;
-            while ((np = node.prev) != p && np != null)
-                (p = np).next = node;   // stale
-            if (whead == p) {
+        for (int spins = -1;;) {
+            WNode h, np, pp; int ps;
+            if ((h = whead) == p) {
+                if (spins < 0)
+                    spins = HEAD_SPINS;
+                else if (spins < MAX_HEAD_SPINS)
+                    spins <<= 1;
                 for (int k = spins;;) { // spin at head
+                    long s, ns;
                     if (((s = state) & ABITS) == 0L) {
-                        if (U.compareAndSwapLong(this, STATE, s, ns = s+WBIT)) {
+                        if (U.compareAndSwapLong(this, STATE, s,
+                                                 ns = s + WBIT)) {
                             whead = node;
                             node.prev = null;
                             return ns;
@@ -1081,10 +1083,21 @@ public class StampedLock implements java.io.Serializable {
                              --k <= 0)
                         break;
                 }
-                if (spins < MAX_HEAD_SPINS)
-                    spins <<= 1;
             }
-            if ((ps = p.status) == 0)
+            else if (h != null) { // help release stale waiters
+                WNode c; Thread w;
+                while ((c = h.cowait) != null) {
+                    if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null)
+                        U.unpark(w);
+                }
+            }
+            if (whead == h) {
+                if ((np = node.prev) != p) {
+                    if (np != null)
+                        (p = np).next = node;   // stale
+                }
+                else if ((ps = p.status) == 0)
                     U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
                 else if (ps == CANCELLED) {
                     if ((pp = p.prev) != null) {
@@ -1099,11 +1112,11 @@ public class StampedLock implements java.io.Serializable {
                     else if ((time = deadline - System.nanoTime()) <= 0L)
                         return cancelWaiter(node, node, false);
                     Thread wt = Thread.currentThread();
-                U.putObject(wt, PARKBLOCKER, this); // emulate LockSupport.park
+                    U.putObject(wt, PARKBLOCKER, this);
                     node.thread = wt;
-                if (node.prev == p && p.status == WAITING && // recheck
-                    (p != whead || (state & ABITS) != 0L))
-                    U.park(false, time);
+                    if (p.status < 0 && (p != h || (state & ABITS) != 0L) &&
+                        whead == h && node.prev == p)
+                        U.park(false, time);  // emulate LockSupport.park
                     node.thread = null;
                     U.putObject(wt, PARKBLOCKER, null);
                     if (interruptible && Thread.interrupted())
@@ -1111,6 +1124,7 @@ public class StampedLock implements java.io.Serializable {
                 }
             }
         }
+    }
 
     /**
      * See above for explanation.
@@ -1122,115 +1136,134 @@ public class StampedLock implements java.io.Serializable {
      * @return next state, or INTERRUPTED
      */
     private long acquireRead(boolean interruptible, long deadline) {
-        WNode node = null, group = null, p;
+        WNode node = null, p;
         for (int spins = -1;;) {
-            for (;;) {
-                long s, m, ns; WNode h, q; Thread w; // anti-barging guard
-                if (group == null && (h = whead) != null &&
-                    (q = h.next) != null && q.mode != RMODE)
-                    break;
+            WNode h;
+            if ((h = whead) == (p = wtail)) {
+                for (long m, s, ns;;) {
                     if ((m = (s = state) & ABITS) < RFULL ?
                         U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
-                    (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
-                    if (group != null) {  // help release others
-                        for (WNode r = group;;) {
-                            if ((w = r.thread) != null) {
-                                r.thread = null;
-                                U.unpark(w);
+                        (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L))
+                        return ns;
+                    else if (m >= WBIT) {
+                        if (spins > 0) {
+                            if (LockSupport.nextSecondarySeed() >= 0)
+                                --spins;
                         }
-                            if ((r = group.cowait) == null)
+                        else {
+                            if (spins == 0) {
+                                WNode nh = whead, np = wtail;
+                                if ((nh == h && np == p) || (h = nh) != (p = np))
                                     break;
-                            U.compareAndSwapObject(group, WCOWAIT, r, r.cowait);
                             }
+                            spins = SPINS;
                         }
-                    return ns;
                     }
-                if (m >= WBIT)
-                    break;
                 }
-            if (spins > 0) {
-                if (LockSupport.nextSecondarySeed() >= 0)
-                    --spins;
             }
-            else if ((p = wtail) == null) {
-                WNode h = new WNode(WMODE, null);
-                if (U.compareAndSwapObject(this, WHEAD, null, h))
-                    wtail = h;
+            if (p == null) { // initialize queue
+                WNode hd = new WNode(WMODE, null);
+                if (U.compareAndSwapObject(this, WHEAD, null, hd))
+                    wtail = hd;
             }
-            else if (spins < 0)
-                spins = (p == whead) ? SPINS : 0;
             else if (node == null)
-                node = new WNode(WMODE, p);
-            else if (node.prev != p)
+                node = new WNode(RMODE, p);
+            else if (h == p || p.mode != RMODE) {
+                if (node.prev != p)
                     node.prev = p;
-            else if (p.mode == RMODE && p != whead) {
-                WNode pp = p.prev;  // become co-waiter with group p
-                if (pp != null && p == wtail &&
-                    U.compareAndSwapObject(p, WCOWAIT,
-                                           node.cowait = p.cowait, node)) {
-                    node.thread = Thread.currentThread();
-                    for (long time;;) {
+                else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
+                    p.next = node;
+                    break;
+                }
+            }
+            else if (!U.compareAndSwapObject(p, WCOWAIT,
+                                             node.cowait = p.cowait, node))
+                node.cowait = null;
+            else {
+                for (;;) {
+                    WNode pp, c; Thread w;
+                    if ((h = whead) != null && (c = h.cowait) != null &&
+                        U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null) // help release
+                        U.unpark(w);
+                    if (h == (pp = p.prev) || h == p || pp == null) {
+                        long m, s, ns;
+                        do {
+                            if ((m = (s = state) & ABITS) < RFULL ?
+                                U.compareAndSwapLong(this, STATE, s,
+                                                     ns = s + RUNIT) :
+                                (m < WBIT &&
+                                 (ns = tryIncReaderOverflow(s)) != 0L))
+                                return ns;
+                        } while (m < WBIT);
+                    }
+                    if (whead == h && p.prev == pp) {
+                        long time;
+                        if (pp == null || h == p || p.status > 0) {
+                            node = null; // throw away
+                            break;
+                        }
                         if (deadline == 0L)
                             time = 0L;
                         else if ((time = deadline - System.nanoTime()) <= 0L)
                             return cancelWaiter(node, p, false);
-                        if (node.thread == null)
-                            break;
-                        if (p.prev != pp || p.status == CANCELLED ||
-                            p == whead || p.prev != pp) {
-                            node.thread = null;
-                            break;
-                        }
                         Thread wt = Thread.currentThread();
                         U.putObject(wt, PARKBLOCKER, this);
-                        if (node.thread == null) // must recheck
-                            break;
+                        node.thread = wt;
+                        if ((h != pp || (state & ABITS) == WBIT) &&
+                            whead == h && p.prev == pp)
                             U.park(false, time);
+                        node.thread = null;
                         U.putObject(wt, PARKBLOCKER, null);
                         if (interruptible && Thread.interrupted())
                             return cancelWaiter(node, p, true);
                     }
-                    group = p;
                 }
-                node = null; // throw away
-            }
-            else if (U.compareAndSwapObject(this, WTAIL, p, node)) {
-                p.next = node;
-                break;
             }
         }
 
-        for (int spins = SPINS;;) {
-            WNode np, pp, r; int ps; long m, s, ns; Thread w;
-            while ((np = node.prev) != p && np != null)
-                (p = np).next = node;
-            if (whead == p) {
-                for (int k = spins;;) {
-                    if ((m = (s = state) & ABITS) != WBIT) {
-                        if (m < RFULL ?
-                            U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT):
-                            (ns = tryIncReaderOverflow(s)) != 0L) {
+        for (int spins = -1;;) {
+            WNode h, np, pp; int ps;
+            if ((h = whead) == p) {
+                if (spins < 0)
+                    spins = HEAD_SPINS;
+                else if (spins < MAX_HEAD_SPINS)
+                    spins <<= 1;
+                for (int k = spins;;) { // spin at head
+                    long m, s, ns;
+                    if ((m = (s = state) & ABITS) < RFULL ?
+                        U.compareAndSwapLong(this, STATE, s, ns = s + RUNIT) :
+                        (m < WBIT && (ns = tryIncReaderOverflow(s)) != 0L)) {
+                        WNode c; Thread w;
                         whead = node;
                         node.prev = null;
-                            while ((r = node.cowait) != null) {
+                        while ((c = node.cowait) != null) {
                             if (U.compareAndSwapObject(node, WCOWAIT,
-                                                           r, r.cowait) &&
-                                    (w = r.thread) != null) {
-                                    r.thread = null;
-                                    U.unpark(w); // release co-waiter
-                                }
+                                                       c, c.cowait) &&
+                                (w = c.thread) != null)
+                                U.unpark(w);
                         }
                         return ns;
                     }
-                    }
-                    else if (LockSupport.nextSecondarySeed() >= 0 &&
-                             --k <= 0)
+                    else if (m >= WBIT &&
+                             LockSupport.nextSecondarySeed() >= 0 && --k <= 0)
                         break;
                 }
-                if (spins < MAX_HEAD_SPINS)
-                    spins <<= 1;
             }
-            if ((ps = p.status) == 0)
+            else if (h != null) {
+                WNode c; Thread w;
+                while ((c = h.cowait) != null) {
+                    if (U.compareAndSwapObject(h, WCOWAIT, c, c.cowait) &&
+                        (w = c.thread) != null)
+                        U.unpark(w);
+                }
+            }
+            if (whead == h) {
+                if ((np = node.prev) != p) {
+                    if (np != null)
+                        (p = np).next = node;   // stale
+                }
+                else if ((ps = p.status) == 0)
                     U.compareAndSwapInt(p, WSTATUS, 0, WAITING);
                 else if (ps == CANCELLED) {
                     if ((pp = p.prev) != null) {
@@ -1247,8 +1280,9 @@ public class StampedLock implements java.io.Serializable {
                     Thread wt = Thread.currentThread();
                     U.putObject(wt, PARKBLOCKER, this);
                     node.thread = wt;
-                if (node.prev == p && p.status == WAITING &&
-                    (p != whead || (state & ABITS) != WBIT))
+                    if (p.status < 0 &&
+                        (p != h || (state & ABITS) == WBIT) &&
+                        whead == h && node.prev == p)
                         U.park(false, time);
                     node.thread = null;
                     U.putObject(wt, PARKBLOCKER, null);
@@ -1257,6 +1291,7 @@ public class StampedLock implements java.io.Serializable {
                 }
             }
         }
+    }
 
     /**
      * If node non-null, forces cancel status and unsplices it from
@@ -1278,22 +1313,19 @@ public class StampedLock implements java.io.Serializable {
         if (node != null && group != null) {
             Thread w;
             node.status = CANCELLED;
-            node.thread = null;
             // unsplice cancelled nodes from group
             for (WNode p = group, q; (q = p.cowait) != null;) {
-                if (q.status == CANCELLED)
-                    U.compareAndSwapObject(p, WNEXT, q, q.next);
+                if (q.status == CANCELLED) {
+                    U.compareAndSwapObject(p, WCOWAIT, q, q.cowait);
+                    p = group; // restart
+                }
                 else
                     p = q;
             }
             if (group == node) {
-                WNode r; // detach and wake up uncancelled co-waiters
-                while ((r = node.cowait) != null) {
-                    if (U.compareAndSwapObject(node, WCOWAIT, r, r.cowait) &&
-                        (w = r.thread) != null) {
-                        r.thread = null;
-                        U.unpark(w);
-                    }
+                for (WNode r = group.cowait; r != null; r = r.cowait) {
+                    if ((w = r.thread) != null)
+                        U.unpark(w);       // wake up uncancelled co-waiters
                 }
                 for (WNode pred = node.prev; pred != null; ) { // unsplice
                     WNode succ, pp;        // find valid successor

test/java/util/concurrent/locks/StampedLock/ReadersUnlockAfterWriteUnlock.java

+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @run main/othervm/timeout=60 ReadersUnlockAfterWriteUnlock
+ * @bug 8023234
+ * @summary StampedLock serializes readers on writer unlock
+ * @author Dmitry Chyuko
+ * @author Aleksey Shipilev
+ */
+
+import java.util.concurrent.CyclicBarrier;
+import java.util.concurrent.locks.StampedLock;
+
+public class ReadersUnlockAfterWriteUnlock {
+    static final int RNUM = 2;
+    static final StampedLock sl = new StampedLock();
+    static volatile boolean isDone;
+
+    static CyclicBarrier iterationStart = new CyclicBarrier(RNUM + 1);
+    static CyclicBarrier readersHaveLocks = new CyclicBarrier(RNUM);
+    static CyclicBarrier writerHasLock = new CyclicBarrier(RNUM + 1);
+
+    static class Reader extends Thread {
+        final String name;
+        Reader(String name) {
+            super();
+            this.name = name;
+        }
+        public void run() {
+            while (!isDone && !isInterrupted()) {
+                try {
+                    iterationStart.await();
+                    writerHasLock.await();
+                    long rs = sl.readLock();
+
+                    // single reader blocks here indefinitely if readers
+                    // are serialized
+                    readersHaveLocks.await();
+
+                    sl.unlockRead(rs);
+                } catch (Exception e) {
+                    throw new IllegalStateException(e);
+                }
+            }
+        }
+    }
+
+    public static void main(String[] args) throws InterruptedException {
+        for (int r = 0 ; r < RNUM; ++r) {
+            new Reader("r" + r).start();
+        }
+        int i;
+        for (i = 0; i < 1024; ++i) {
+            try {
+                iterationStart.await();
+                long ws = sl.writeLock();
+                writerHasLock.await();
+                Thread.sleep(10);
+                sl.unlockWrite(ws);
+            } catch (Exception e) {
+                throw new IllegalStateException(e);
+            }
+        }
+        isDone = true;
+    }
+
+}