[PATCH 1/4] seqlock: introduce scoped_seqlock_read() and scoped_seqlock_read_irqsave()

[Oleg Nesterov <oleg@redhat.com>]

The read_seqbegin/need_seqretry/done_seqretry API is cumbersome and
error prone. With the new helper the "typical" code like

	int seq, nextseq;
	unsigned long flags;

	nextseq = 0;
	do {
		seq = nextseq;
		flags = read_seqbegin_or_lock_irqsave(&seqlock, &seq);

		// read-side critical section

		nextseq = 1;
	} while (need_seqretry(&seqlock, seq));
	done_seqretry_irqrestore(&seqlock, seq, flags);

can be rewritten as

	scoped_seqlock_read_irqsave (&seqlock) {
		// read-side critical section
	}

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/seqlock.h | 61 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 61 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 5ce48eab7a2a..9012702fd0a8 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -1209,4 +1209,65 @@ done_seqretry_irqrestore(seqlock_t *lock, int seq, unsigned long flags)
 	if (seq & 1)
 		read_sequnlock_excl_irqrestore(lock, flags);
 }
+
+/* internal helper for scoped_seqlock_read/scoped_seqlock_read_irqsave */
+static inline int
+scoped_seqlock_read_retry(seqlock_t *lock, int *seq, unsigned long *flags)
+{
+	int retry = 0;
+
+	if (*seq & 1) {
+		if (flags)
+			read_sequnlock_excl_irqrestore(lock, *flags);
+		else
+			read_sequnlock_excl(lock);
+	} else if (read_seqretry(lock, *seq)) {
+		retry = *seq = 1;
+		if (flags)
+			read_seqlock_excl_irqsave(lock, *flags);
+		else
+			read_seqlock_excl(lock);
+	}
+
+	return retry;
+}
+
+#define __scoped_seqlock_read(lock, lockless, seq)	\
+	for (int lockless = 1, seq = read_seqbegin(lock);		\
+	     lockless || scoped_seqlock_read_retry(lock, &seq, NULL);	\
+	     lockless = 0)
+
+/**
+ * scoped_seqlock_read(lock) - execute the read side critical section
+ *                             without manual sequence counter handling
+ *                             or calls to other helpers
+ * @lock: pointer to the seqlock_t protecting the data
+ *
+ * Example:
+ *
+ *	scoped_seqlock_read(&lock) {
+ *		// read-side critical section
+ *	}
+ *
+ * Starts with a lockless pass first. If it fails, restarts the critical
+ * section with the lock held.
+ *
+ * The critical section must not contain control flow that escapes the loop.
+ */
+#define scoped_seqlock_read(lock)	\
+	__scoped_seqlock_read(lock, __UNIQUE_ID(lockless), __UNIQUE_ID(seq))
+
+#define __scoped_seqlock_read_irqsave(lock, s)	\
+	for (struct { int lockless, seq; ulong flags; } s = { 1, read_seqbegin(lock) }; \
+	     s.lockless || scoped_seqlock_read_retry(lock, &s.seq, &s.flags);		\
+	     s.lockless = 0)
+
+/**
+ * scoped_seqlock_read_irqsave(lock) - same as scoped_seqlock_read() but
+ *                                     disables irqs on a locking pass
+ * @lock: pointer to the seqlock_t protecting the data
+ */
+#define scoped_seqlock_read_irqsave(lock)	\
+	__scoped_seqlock_read_irqsave(lock, __UNIQUE_ID(s))
+
 #endif /* __LINUX_SEQLOCK_H */
-- 
2.25.1.362.g51ebf55