rfifolock: add recursive FIFO lock

QemuMutex does not guarantee fairness and cannot be acquired
recursively:

Fairness means each locker gets a turn and the scheduler cannot cause
starvation.

Recursive locking is useful for composition, it allows a sequence of
locking operations to be invoked atomically by acquiring the lock around
them.

This patch adds RFifoLock, a recursive lock that guarantees FIFO order.
Its first user is added in the next patch.

RFifoLock has one additional feature: it can be initialized with an
optional contention callback.  The callback is invoked whenever a thread
must wait for the lock.  For example, it can be used to poke the current
owner so that they release the lock soon.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

util/Makefile.objs

@@ -14,3 +14,4 @@ util-obj-y += crc32c.o
 util-obj-y += throttle.o
 util-obj-y += getauxval.o
 util-obj-y += readline.o
+util-obj-y += rfifolock.o

util/rfifolock.c

@@ -0,0 +1,78 @@
+/*
+ * Recursive FIFO lock
+ *
+ * Copyright Red Hat, Inc. 2013
+ *
+ * Authors:
+ *  Stefan Hajnoczi   <stefanha@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU LGPL, version 2 or later.
+ * See the COPYING.LIB file in the top-level directory.
+ *
+ */
+
+#include <assert.h>
+#include "qemu/rfifolock.h"
+
+void rfifolock_init(RFifoLock *r, void (*cb)(void *), void *opaque)
+{
+    qemu_mutex_init(&r->lock);
+    r->head = 0;
+    r->tail = 0;
+    qemu_cond_init(&r->cond);
+    r->nesting = 0;
+    r->cb = cb;
+    r->cb_opaque = opaque;
+}
+
+void rfifolock_destroy(RFifoLock *r)
+{
+    qemu_cond_destroy(&r->cond);
+    qemu_mutex_destroy(&r->lock);
+}
+
+/*
+ * Theory of operation:
+ *
+ * In order to ensure FIFO ordering, implement a ticketlock.  Threads acquiring
+ * the lock enqueue themselves by incrementing the tail index.  When the lock
+ * is unlocked, the head is incremented and waiting threads are notified.
+ *
+ * Recursive locking does not take a ticket since the head is only incremented
+ * when the outermost recursive caller unlocks.
+ */
+void rfifolock_lock(RFifoLock *r)
+{
+    qemu_mutex_lock(&r->lock);
+
+    /* Take a ticket */
+    unsigned int ticket = r->tail++;
+
+    if (r->nesting > 0 && qemu_thread_is_self(&r->owner_thread)) {
+        r->tail--; /* put ticket back, we're nesting */
+    } else {
+        while (ticket != r->head) {
+            /* Invoke optional contention callback */
+            if (r->cb) {
+                r->cb(r->cb_opaque);
+            }
+            qemu_cond_wait(&r->cond, &r->lock);
+        }
+    }
+
+    qemu_thread_get_self(&r->owner_thread);
+    r->nesting++;
+    qemu_mutex_unlock(&r->lock);
+}
+
+void rfifolock_unlock(RFifoLock *r)
+{
+    qemu_mutex_lock(&r->lock);
+    assert(r->nesting > 0);
+    assert(qemu_thread_is_self(&r->owner_thread));
+    if (--r->nesting == 0) {
+        r->head++;
+        qemu_cond_broadcast(&r->cond);
+    }
+    qemu_mutex_unlock(&r->lock);
+}