Subject: seqlock: Provide seq_spin_* functions
From: Thomas Gleixner <tglx@linutronix.de>
Date: Mon, 27 Feb 2012 17:55:11 +0100

In some cases it's desirable to lock the seqlock w/o changing the
seqcount. Provide functions for this, so we can avoid open coded
constructs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable-rt@vger.kernel.org
---
 include/linux/seqlock.h |   64 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 64 insertions(+)

Index: rt/include/linux/seqlock.h
===================================================================
--- rt.orig/include/linux/seqlock.h
+++ rt/include/linux/seqlock.h
@@ -188,6 +188,19 @@ static inline unsigned read_seqretry(con
 }
 
 /*
+ * Ditto w/o barriers
+ */
+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.
@@ -247,4 +260,55 @@ write_sequnlock_irqrestore(seqlock_t *sl
 	spin_unlock_irqrestore(&sl->lock, flags);
 }
 
+/*
+ * Instead of open coding a spinlock and a seqcount, the following
+ * functions allow to serialize on the seqlock w/o touching seqcount.
+ */
+static inline void seq_spin_lock(seqlock_t *sl)
+{
+	spin_lock(&sl->lock);
+}
+
+static inline int seq_spin_trylock(seqlock_t *sl)
+{
+	return spin_trylock(&sl->lock);
+}
+
+static inline void seq_spin_unlock(seqlock_t *sl)
+{
+	spin_unlock(&sl->lock);
+}
+
+static inline void assert_seq_spin_locked(seqlock_t *sl)
+{
+	assert_spin_locked(&sl->lock);
+}
+
+static inline void seq_spin_lock_nested(seqlock_t *sl, int subclass)
+{
+	spin_lock_nested(&sl->lock, subclass);
+}
+
+/*
+ * For writers which need to take/release the lock w/o updating seqcount for
+ * whatever reasons the following functions allow to update the count
+ * after the lock has been acquired or before it is released.
+ */
+static inline void write_seqlock_begin(seqlock_t *sl)
+{
+	assert_spin_locked(&sl->lock);
+	write_seqcount_begin(&sl->seqcount);
+}
+
+static inline void write_seqlock_end(seqlock_t *sl)
+{
+	assert_spin_locked(&sl->lock);
+	write_seqcount_end(&sl->seqcount);
+}
+
+static inline void write_seqlock_barrier(seqlock_t *sl)
+{
+	write_seqcount_barrier(&sl->seqcount);
+}
+
 #endif /* __LINUX_SEQLOCK_H */