--- /dev/null
+#ifndef _LWK_SEQLOCK_H
+#define _LWK_SEQLOCK_H
+/*
+ * Reader/writer consistent mechanism without starving writers. This type of
+ * lock for data where the reader wants a consitent 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 <lwk/spinlock.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 { 0, SPIN_LOCK_UNLOCKED }
+#define seqlock_init(x) do { *(x) = (seqlock_t) SEQLOCK_UNLOCKED; } while (0)
+
+
+/* Lock out other writers and update the count.
+ * Acts like a normal spin_lock/unlock.
+ */
+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 = sl->sequence;
+ smp_rmb();
+ return ret;
+}
+
+/* Test if reader processed invalid data.
+ * If initial values is odd,
+ * then writer had already started when section was entered
+ * If sequence value changed
+ * then writer changed data while in section
+ *
+ * Using xor saves one conditional branch.
+ */
+static __always_inline int read_seqretry(const seqlock_t *sl, unsigned iv)
+{
+ smp_rmb();
+ 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.
+ */
+#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 /* _LWK_SEQLOCK_H */
