[Qemu-devel] [PATCH v4 1/4] seqlock: introduce read-write seqlock

[Liu Ping Fan <qemulist@gmail.com>]

This lets the read-side access run outside the BQL.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 include/qemu/seqlock.h | 72 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 include/qemu/seqlock.h

diff --git a/include/qemu/seqlock.h b/include/qemu/seqlock.h
new file mode 100644
index 0000000..3ff118a
--- /dev/null
+++ b/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
-- 
1.8.1.4