1 files changed, 478 insertions, 0 deletions
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
new file mode 100644
index 000000000..5f68d0a39
--- /dev/null
+++ b/include/linux/seqlock.h
@@ -0,0 +1,478 @@
+#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. There are two types
+ * of readers:
+ * 1. Sequence readers which never block a writer but they may have to retry
+ *    if a writer is in progress by detecting change in sequence number.
+ *    Writers do not wait for a sequence reader.
+ * 2. Locking readers which will wait if a writer or another locking reader
+ *    is in progress. A locking reader in progress will also block a writer
+ *    from going forward. Unlike the regular rwlock, the read lock here is
+ *    exclusive so that only one locking reader can get it.
+ *
+ * This is not as cache friendly as brlock. Also, this may not work well
+ * for data that contains pointers, because any writer could
+ * invalidate a pointer that a reader was following.
+ *
+ * Expected non-blocking 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 <linux/lockdep.h>
+#include <asm/processor.h>
+
+/*
+ * 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;
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+	struct lockdep_map dep_map;
+#endif
+} seqcount_t;
+
+static inline void __seqcount_init(seqcount_t *s, const char *name,
+					  struct lock_class_key *key)
+{
+	/*
+	 * Make sure we are not reinitializing a held lock:
+	 */
+	lockdep_init_map(&s->dep_map, name, key, 0);
+	s->sequence = 0;
+}
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+# define SEQCOUNT_DEP_MAP_INIT(lockname) \
+		.dep_map = { .name = #lockname } \
+
+# define seqcount_init(s)				\
+	do {						\
+		static struct lock_class_key __key;	\
+		__seqcount_init((s), #s, &__key);	\
+	} while (0)
+
+static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
+{
+	seqcount_t *l = (seqcount_t *)s;
+	unsigned long flags;
+
+	local_irq_save(flags);
+	seqcount_acquire_read(&l->dep_map, 0, 0, _RET_IP_);
+	seqcount_release(&l->dep_map, 1, _RET_IP_);
+	local_irq_restore(flags);
+}
+
+#else
+# define SEQCOUNT_DEP_MAP_INIT(lockname)
+# define seqcount_init(s) __seqcount_init(s, NULL, NULL)
+# define seqcount_lockdep_reader_access(x)
+#endif
+
+#define SEQCNT_ZERO(lockname) { .sequence = 0, SEQCOUNT_DEP_MAP_INIT(lockname)}
+
+
+/**
+ * __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 = READ_ONCE(s->sequence);
+	if (unlikely(ret & 1)) {
+		cpu_relax();
+		goto repeat;
+	}
+	return ret;
+}
+
+/**
+ * raw_read_seqcount - Read the raw seqcount
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * raw_read_seqcount opens a read critical section of the given
+ * seqcount without any lockdep checking and without checking or
+ * masking the LSB. Calling code is responsible for handling that.
+ */
+static inline unsigned raw_read_seqcount(const seqcount_t *s)
+{
+	unsigned ret = READ_ONCE(s->sequence);
+	smp_rmb();
+	return ret;
+}
+
+/**
+ * raw_read_seqcount_begin - start seq-read critical section w/o lockdep
+ * @s: pointer to seqcount_t
+ * Returns: count to be passed to read_seqcount_retry
+ *
+ * raw_read_seqcount_begin opens a read critical section of the given
+ * seqcount, but without any lockdep checking. Validity of the critical
+ * section is tested by checking read_seqcount_retry function.
+ */
+static inline unsigned raw_read_seqcount_begin(const seqcount_t *s)
+{
+	unsigned ret = __read_seqcount_begin(s);
+	smp_rmb();
+	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)
+{
+	seqcount_lockdep_reader_access(s);
+	return raw_read_seqcount_begin(s);
+}
+
+/**
+ * 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 = READ_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);
+}
+
+
+
+static inline void raw_write_seqcount_begin(seqcount_t *s)
+{
+	s->sequence++;
+	smp_wmb();
+}
+
+static inline void raw_write_seqcount_end(seqcount_t *s)
+{
+	smp_wmb();
+	s->sequence++;
+}
+
+/*
+ * raw_write_seqcount_latch - redirect readers to even/odd copy
+ * @s: pointer to seqcount_t
+ */
+static inline void raw_write_seqcount_latch(seqcount_t *s)
+{
+       smp_wmb();      /* prior stores before incrementing "sequence" */
+       s->sequence++;
+       smp_wmb();      /* increment "sequence" before following stores */
+}
+
+/*
+ * Sequence counter only version assumes that callers are using their
+ * own mutexing.
+ */
+static inline void write_seqcount_begin_nested(seqcount_t *s, int subclass)
+{
+	raw_write_seqcount_begin(s);
+	seqcount_acquire(&s->dep_map, subclass, 0, _RET_IP_);
+}
+
+static inline void write_seqcount_begin(seqcount_t *s)
+{
+	write_seqcount_begin_nested(s, 0);
+}
+
+static inline void write_seqcount_end(seqcount_t *s)
+{
+	seqcount_release(&s->dep_map, 1, _RET_IP_);
+	raw_write_seqcount_end(s);
+}
+
+/**
+ * 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;
+}
+
+typedef struct {
+	struct seqcount seqcount;
+	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)			\
+	{						\
+		.seqcount = SEQCNT_ZERO(lockname),	\
+		.lock =	__SPIN_LOCK_UNLOCKED(lockname)	\
+	}
+
+#define seqlock_init(x)					\
+	do {						\
+		seqcount_init(&(x)->seqcount);		\
+		spin_lock_init(&(x)->lock);		\
+	} while (0)
+
+#define DEFINE_SEQLOCK(x) \
+		seqlock_t x = __SEQLOCK_UNLOCKED(x)
+
+/*
+ * Read side functions for starting and finalizing a read side section.
+ */
+static inline unsigned read_seqbegin(const seqlock_t *sl)
+{
+	return read_seqcount_begin(&sl->seqcount);
+}
+
+static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start)
+{
+	return read_seqcount_retry(&sl->seqcount, start);
+}
+
+/*
+ * 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);
+	write_seqcount_begin(&sl->seqcount);
+}
+
+static inline void write_sequnlock(seqlock_t *sl)
+{
+	write_seqcount_end(&sl->seqcount);
+	spin_unlock(&sl->lock);
+}
+
+static inline void write_seqlock_bh(seqlock_t *sl)
+{
+	spin_lock_bh(&sl->lock);
+	write_seqcount_begin(&sl->seqcount);
+}
+
+static inline void write_sequnlock_bh(seqlock_t *sl)
+{
+	write_seqcount_end(&sl->seqcount);
+	spin_unlock_bh(&sl->lock);
+}
+
+static inline void write_seqlock_irq(seqlock_t *sl)
+{
+	spin_lock_irq(&sl->lock);
+	write_seqcount_begin(&sl->seqcount);
+}
+
+static inline void write_sequnlock_irq(seqlock_t *sl)
+{
+	write_seqcount_end(&sl->seqcount);
+	spin_unlock_irq(&sl->lock);
+}
+
+static inline unsigned long __write_seqlock_irqsave(seqlock_t *sl)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sl->lock, flags);
+	write_seqcount_begin(&sl->seqcount);
+	return flags;
+}
+
+#define write_seqlock_irqsave(lock, flags)				\
+	do { flags = __write_seqlock_irqsave(lock); } while (0)
+
+static inline void
+write_sequnlock_irqrestore(seqlock_t *sl, unsigned long flags)
+{
+	write_seqcount_end(&sl->seqcount);
+	spin_unlock_irqrestore(&sl->lock, flags);
+}
+
+/*
+ * A locking reader exclusively locks out other writers and locking readers,
+ * but doesn't update the sequence number. Acts like a normal spin_lock/unlock.
+ * Don't need preempt_disable() because that is in the spin_lock already.
+ */
+static inline void read_seqlock_excl(seqlock_t *sl)
+{
+	spin_lock(&sl->lock);
+}
+
+static inline void read_sequnlock_excl(seqlock_t *sl)
+{
+	spin_unlock(&sl->lock);
+}
+
+/**
+ * read_seqbegin_or_lock - begin a sequence number check or locking block
+ * @lock: sequence lock
+ * @seq : sequence number to be checked
+ *
+ * First try it once optimistically without taking the lock. If that fails,
+ * take the lock. The sequence number is also used as a marker for deciding
+ * whether to be a reader (even) or writer (odd).
+ * N.B. seq must be initialized to an even number to begin with.
+ */
+static inline void read_seqbegin_or_lock(seqlock_t *lock, int *seq)
+{
+	if (!(*seq & 1))	/* Even */
+		*seq = read_seqbegin(lock);
+	else			/* Odd */
+		read_seqlock_excl(lock);
+}
+
+static inline int need_seqretry(seqlock_t *lock, int seq)
+{
+	return !(seq & 1) && read_seqretry(lock, seq);
+}
+
+static inline void done_seqretry(seqlock_t *lock, int seq)
+{
+	if (seq & 1)
+		read_sequnlock_excl(lock);
+}
+
+static inline void read_seqlock_excl_bh(seqlock_t *sl)
+{
+	spin_lock_bh(&sl->lock);
+}
+
+static inline void read_sequnlock_excl_bh(seqlock_t *sl)
+{
+	spin_unlock_bh(&sl->lock);
+}
+
+static inline void read_seqlock_excl_irq(seqlock_t *sl)
+{
+	spin_lock_irq(&sl->lock);
+}
+
+static inline void read_sequnlock_excl_irq(seqlock_t *sl)
+{
+	spin_unlock_irq(&sl->lock);
+}
+
+static inline unsigned long __read_seqlock_excl_irqsave(seqlock_t *sl)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&sl->lock, flags);
+	return flags;
+}
+
+#define read_seqlock_excl_irqsave(lock, flags)				\
+	do { flags = __read_seqlock_excl_irqsave(lock); } while (0)
+
+static inline void
+read_sequnlock_excl_irqrestore(seqlock_t *sl, unsigned long flags)
+{
+	spin_unlock_irqrestore(&sl->lock, flags);
+}
+
+static inline unsigned long
+read_seqbegin_or_lock_irqsave(seqlock_t *lock, int *seq)
+{
+	unsigned long flags = 0;
+
+	if (!(*seq & 1))	/* Even */
+		*seq = read_seqbegin(lock);
+	else			/* Odd */
+		read_seqlock_excl_irqsave(lock, flags);
+
+	return flags;
+}
+
+static inline void
+done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
+{
+	if (seq & 1)
+		read_sequnlock_excl_irqrestore(lock, flags);
+}
+#endif /* __LINUX_SEQLOCK_H */