seqlock: introduce read-write seqlock

Seqlock implementation for QEMU. Usage idiom

reader:
    do {
        start = seqlock_read_begin(&sl);
        ...
    } while (seqlock_read_retry(&sl, start));

writer:
    seqlock_write_lock(&sl);
    ...
    seqlock_write_unlock(&sl);

initialization:
    seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)

    mutex could be NULL if the caller will provide its own protection
    for concurrent write sides (typically using the BQL).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/seqlock.h

@@ -0,0 +1,72 @@
+/*
+ * Seqlock implementation for QEMU
+ *
+ * Copyright Red Hat, Inc. 2013
+ *
+ * Author:
+ *  Paolo Bonzini <pbonzini@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ *
+ */
+#ifndef QEMU_SEQLOCK_H
+#define QEMU_SEQLOCK_H 1
+
+#include <qemu/atomic.h>
+#include <qemu/thread.h>
+
+typedef struct QemuSeqLock QemuSeqLock;
+
+struct QemuSeqLock {
+    QemuMutex *mutex;
+    unsigned sequence;
+};
+
+static inline void seqlock_init(QemuSeqLock *sl, QemuMutex *mutex)
+{
+    sl->mutex = mutex;
+    sl->sequence = 0;
+}
+
+/* Lock out other writers and update the count.  */
+static inline void seqlock_write_lock(QemuSeqLock *sl)
+{
+    if (sl->mutex) {
+        qemu_mutex_lock(sl->mutex);
+    }
+    ++sl->sequence;
+
+    /* Write sequence before updating other fields.  */
+    smp_wmb();
+}
+
+static inline void seqlock_write_unlock(QemuSeqLock *sl)
+{
+    /* Write other fields before finalizing sequence.  */
+    smp_wmb();
+
+    ++sl->sequence;
+    if (sl->mutex) {
+        qemu_mutex_unlock(sl->mutex);
+    }
+}
+
+static inline unsigned seqlock_read_begin(QemuSeqLock *sl)
+{
+    /* Always fail if a write is in progress.  */
+    unsigned ret = sl->sequence & ~1;
+
+    /* Read sequence before reading other fields.  */
+    smp_rmb();
+    return ret;
+}
+
+static int seqlock_read_retry(const QemuSeqLock *sl, unsigned start)
+{
+    /* Read other fields before reading final sequence.  */
+    smp_rmb();
+    return unlikely(sl->sequence != start);
+}
+
+#endif