[tip:sched/core] seqlock: Add irqsave variant of read_seqbegin_or_lock()

[tip-bot for Rik van Riel <tipbot@zytor.com>]

Commit-ID:  ef8ac06359ddf95431cf6bb04ad2b36fff562328
Gitweb:     http://git.kernel.org/tip/ef8ac06359ddf95431cf6bb04ad2b36fff562328
Author:     Rik van Riel <riel@redhat.com>
AuthorDate: Fri, 12 Sep 2014 09:12:14 -0400
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 19 Sep 2014 12:35:16 +0200

seqlock: Add irqsave variant of read_seqbegin_or_lock()

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: prarit@redhat.com
Cc: oleg@redhat.com
Cc: sgruszka@redhat.com
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Stephen Boyd <sboyd@codeaurora.org>
Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Link: http://lkml.kernel.org/r/1410527535-9814-2-git-send-email-riel@redhat.com
[ Improved the readability of the code a bit. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 include/linux/seqlock.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index cc35963..f5df8f6 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -456,4 +456,23 @@ 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 */