1 files changed, 282 insertions, 0 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
new file mode 100644
index 00000000..600060e2
--- /dev/null
+++ b/include/linux/seqlock.h
@@ -0,0 +1,282 @@
+#ifndef __LINUX_SEQLOCK_H
+#define __LINUX_SEQLOCK_H
+/*
+ * Reader/writer consistent mechanism without starving writers. This type of
+ * lock for data where the reader wants a consistent set of information
+ * and is willing to retry if the information changes.  Readers never
+ * block but they may have to retry if a writer is in
+ * progress. Writers do not wait for readers. 
+ *
+ * This is not as cache friendly as brlock. Also, this will not work
+ * for data that contains pointers, because any writer could
+ * invalidate a pointer that a reader was following.
+ *
+ * Expected reader usage:
+ * 	do {
+ *	    seq = read_seqbegin(&foo);
+ * 	...
+ *      } while (read_seqretry(&foo, seq));
+ *
+ *
+ * On non-SMP the spin locks disappear but the writer still needs
+ * to increment the sequence variables because an interrupt routine could
+ * change the state of the data.
+ *
+ * Based on x86_64 vsyscall gettimeofday 
+ * by Keith Owens and Andrea Arcangeli
+ */
+
+#include <linux/spinlock.h>
+#include <linux/preempt.h>
+#include <asm/processor.h>
+
+typedef struct {
+	unsigned sequence;
+	spinlock_t lock;
+} seqlock_t;
+
+/*
+ * These macros triggered gcc-3.x compile-time problems.  We think these are
+ * OK now.  Be cautious.
+ */
+#define __SEQLOCK_UNLOCKED(lockname) \
+		 { 0, __SPIN_LOCK_UNLOCKED(lockname) }
+
+#define seqlock_init(x)					\
+	do {						\
+		(x)->sequence = 0;			\
+		spin_lock_init(&(x)->lock);		\
+	} while (0)
+
+#define DEFINE_SEQLOCK(x) \
+		seqlock_t x = __SEQLOCK_UNLOCKED(x)
+
+/* Lock out other writers and update the count.
+ * Acts like a normal spin_lock/unlock.
+ * Don't need preempt_disable() because that is in the spin_lock already.
+ */
+static inline void write_seqlock(seqlock_t *sl)
+{
+	spin_lock(&sl->lock);
+	++sl->sequence;
+	smp_wmb();
+}
+
+static inline void write_sequnlock(seqlock_t *sl)
+{
+	smp_wmb();
+	sl->sequence++;
+	spin_unlock(&sl->lock);
+}
+
+static inline int write_tryseqlock(seqlock_t *sl)
+{
+	int ret = spin_trylock(&sl->lock);
+
+	if (ret) {
+		++sl->sequence;
+		smp_wmb();
+	}
+	return ret;
+}
+
+/* Start of read calculation -- fetch last complete writer token */
+static __always_inline unsigned read_seqbegin(const seqlock_t *sl)
+{
+	unsigned ret;
+
+repeat:
+	ret = ACCESS_ONCE(sl->sequence);
+	if (unlikely(ret & 1)) {
+		cpu_relax();
+		goto repeat;
+	}
+	smp_rmb();
+
+	return ret;
+}
+
+/*
+ * Test if reader processed invalid data.
+ *
+ * If sequence value changed then writer changed data while in section.
+ */
+static __always_inline int read_seqretry(const seqlock_t *sl, unsigned start)
+{
+	smp_rmb();
+
+	return unlikely(sl->sequence != start);
+}
+
+
+/*
+ * 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)
+
+/**
+ * __read_seqcount_begin - begin a seq-read critical section (without barrier)
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * __read_seqcount_begin is like read_seqcount_begin, but has no smp_rmb()
+ * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
+ * provided before actually loading any of the variables that are to be
+ * protected in this critical section.
+ *
+ * Use carefully, only in critical code, and comment how the barrier is
+ * provided.
+ */
+static inline unsigned __read_seqcount_begin(const seqcount_t *s)
+{
+	unsigned ret;
+
+repeat:
+	ret = ACCESS_ONCE(s->sequence);
+	if (unlikely(ret & 1)) {
+		cpu_relax();
+		goto repeat;
+	}
+	return ret;
+}
+
+/**
+ * read_seqcount_begin - begin a seq-read critical section
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * read_seqcount_begin opens a read critical section of the given seqcount.
+ * Validity of the critical section is tested by checking read_seqcount_retry
+ * function.
+ */
+static inline unsigned read_seqcount_begin(const seqcount_t *s)
+{
+	unsigned ret = __read_seqcount_begin(s);
+	smp_rmb();
+	return ret;
+}
+
+/**
+ * raw_seqcount_begin - begin a seq-read critical section
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * raw_seqcount_begin opens a read critical section of the given seqcount.
+ * Validity of the critical section is tested by checking read_seqcount_retry
+ * function.
+ *
+ * Unlike read_seqcount_begin(), this function will not wait for the count
+ * to stabilize. If a writer is active when we begin, we will fail the
+ * read_seqcount_retry() instead of stabilizing at the beginning of the
+ * critical section.
+ */
+static inline unsigned raw_seqcount_begin(const seqcount_t *s)
+{
+	unsigned ret = ACCESS_ONCE(s->sequence);
+	smp_rmb();
+	return ret & ~1;
+}
+
+/**
+ * __read_seqcount_retry - end a seq-read critical section (without barrier)
+ * @s: pointer to seqcount_t
+ * @start: count, from read_seqcount_begin
+ * Returns: 1 if retry is required, else 0
+ *
+ * __read_seqcount_retry is like read_seqcount_retry, but has no smp_rmb()
+ * barrier. Callers should ensure that smp_rmb() or equivalent ordering is
+ * provided before actually loading any of the variables that are to be
+ * protected in this critical section.
+ *
+ * Use carefully, only in critical code, and comment how the barrier is
+ * provided.
+ */
+static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start)
+{
+	return unlikely(s->sequence != start);
+}
+
+/**
+ * read_seqcount_retry - end a seq-read critical section
+ * @s: pointer to seqcount_t
+ * @start: count, from read_seqcount_begin
+ * Returns: 1 if retry is required, else 0
+ *
+ * read_seqcount_retry closes a read critical section of the given seqcount.
+ * If the critical section was invalid, it must be ignored (and typically
+ * retried).
+ */
+static inline int read_seqcount_retry(const seqcount_t *s, unsigned start)
+{
+	smp_rmb();
+
+	return __read_seqcount_retry(s, start);
+}
+
+
+/*
+ * 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++;
+}
+
+/**
+ * write_seqcount_barrier - invalidate in-progress read-side seq operations
+ * @s: pointer to seqcount_t
+ *
+ * After write_seqcount_barrier, no read-side seq operations will complete
+ * successfully and see data older than this.
+ */
+static inline void write_seqcount_barrier(seqcount_t *s)
+{
+	smp_wmb();
+	s->sequence+=2;
+}
+
+/*
+ * Possible sw/hw IRQ protected versions of the interfaces.
+ */
+#define write_seqlock_irqsave(lock, flags)				\
+	do { local_irq_save(flags); write_seqlock(lock); } while (0)
+#define write_seqlock_irq(lock)						\
+	do { local_irq_disable();   write_seqlock(lock); } while (0)
+#define write_seqlock_bh(lock)						\
+        do { local_bh_disable();    write_seqlock(lock); } while (0)
+
+#define write_sequnlock_irqrestore(lock, flags)				\
+	do { write_sequnlock(lock); local_irq_restore(flags); } while(0)
+#define write_sequnlock_irq(lock)					\
+	do { write_sequnlock(lock); local_irq_enable(); } while(0)
+#define write_sequnlock_bh(lock)					\
+	do { write_sequnlock(lock); local_bh_enable(); } while(0)
+
+#define read_seqbegin_irqsave(lock, flags)				\
+	({ local_irq_save(flags);   read_seqbegin(lock); })
+
+#define read_seqretry_irqrestore(lock, iv, flags)			\
+	({								\
+		int ret = read_seqretry(lock, iv);			\
+		local_irq_restore(flags);				\
+		ret;							\
+	})
+
+#endif /* __LINUX_SEQLOCK_H */