[PATCH -tip 1/2] seqlock: add irqsave variant of read_seqbegin_or_lock

[riel@redhat.com]

From: Rik van Riel <riel@redhat.com>

There are cases where read_seqbegin_or_lock needs to block irqs,
because the seqlock in question nests inside a lock that is also
be taken from irq context.

Add read_seqbegin_or_lock_irqsave and done_seqretry_irqrestore, which
are almost identical to read_seqbegin_or_lock and done_seqretry.

Signed-off-by: Rik van Riel <riel@redhat.com>
---
 include/linux/seqlock.h | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index cc35963..4364cd3 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -456,4 +456,21 @@ 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 */
-- 
1.9.3