[PATCH] i_size atomic access: infrastructure

From: Daniel McNeil <daniel@osdl.org>

This adds a sequence counter only version of the reader/writer consistent
mechanism to seqlock.h This is used in the second part of this patch give
atomic access to i_size.

include/linux/seqlock.h

@@ -94,6 +94,57 @@ static inline int read_seqretry(const seqlock_t *sl, unsigned iv)
 	return (iv & 1) | (sl->sequence ^ iv);
 }
 
+
+/*
+ * Version using sequence counter only.
+ * This can be used when code has its own mutex protecting the
+ * updating starting before the write_seqcountbeqin() and ending
+ * after the write_seqcount_end().
+ */
+
+typedef struct seqcount {
+	unsigned sequence;
+} seqcount_t;
+
+#define SEQCNT_ZERO { 0 }
+#define seqcount_init(x)	do { *(x) = (seqcount_t) SEQCNT_ZERO; } while (0)
+
+/* Start of read using pointer to a sequence counter only.  */
+static inline unsigned read_seqcount_begin(const seqcount_t *s)
+{
+	unsigned ret = s->sequence;
+	smp_rmb();
+	return ret;
+}
+
+/* Test if reader processed invalid data.
+ * Equivalent to: iv is odd or sequence number has changed.
+ *                (iv & 1) || (*s != iv)
+ * Using xor saves one conditional branch.
+ */
+static inline int read_seqcount_retry(const seqcount_t *s, unsigned iv)
+{
+	smp_rmb();
+	return (iv & 1) | (s->sequence ^ iv);
+}
+
+
+/*
+ * Sequence counter only version assumes that callers are using their
+ * own mutexing.
+ */
+static inline void write_seqcount_begin(seqcount_t *s)
+{
+	s->sequence++;
+	smp_wmb();
+}
+
+static inline void write_seqcount_end(seqcount_t *s)
+{
+	smp_wmb();
+	s->sequence++;
+}
+
 /*
  * Possible sw/hw IRQ protected versions of the interfaces.
  */