[PATCH 2.5.68 1/2] i_size atomic access

[PATCH 2.5.68 1/2] i_size atomic access

[Daniel McNeil]

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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.

re-diff against 2.5.68.

The patch is also available for download from OSDL's patch lifecycle 
manager (PLM):
http://www.osdl.org/cgi-bin/plm?module=patch_info&patch_id=1801

Thanks,
-- 
Daniel McNeil <daniel@osdl.org>

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--- linux-2.5.68/include/linux/seqlock.h	Sat Apr 19 19:49:23 2003
+++ linux-2.5.68-isize/include/linux/seqlock.h	Thu Apr 24 14:30:57 2003
@@ -94,6 +94,57 @@ static inline int read_seqretry(const se
 	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_seqcntbeqin() and ending
+ * after the write_seqcntend().
+ */
+
+typedef struct seqcnt {
+	unsigned sequence;
+} seqcnt_t;
+
+#define SEQCNT_ZERO { 0 }
+#define seqcnt_init(x)	do { *(x) = (seqcnt_t) SEQCNT_ZERO; } while (0)
+
+/* Start of read using pointer to a sequence counter only.  */
+static inline unsigned read_seqcntbegin(const seqcnt_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_seqcntretry(const seqcnt_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_seqcntbegin(seqcnt_t *s)
+{
+	s->sequence++;
+	smp_wmb();			
+}	
+
+static inline void write_seqcntend(seqcnt_t *s) 
+{
+	smp_wmb();
+	s->sequence++;
+}
+
 /*
  * Possible sw/hw IRQ protected versions of the interfaces.
  */