Commit-ID: c4bfa3f5f906aee2e084c5b1fb15caf876338ef8
Gitweb: http://git.kernel.org/tip/c4bfa3f5f906aee2e084c5b1fb15caf876338ef8
Author: Peter Zijlstra <pet...@infradead.org>
AuthorDate: Wed, 17 Jun 2015 14:29:24 +0200
Committer: Thomas Gleixner <t...@linutronix.de>
CommitDate: Fri, 19 Jun 2015 00:09:56 +0200
seqcount: Introduce raw_write_seqcount_barrier()
Introduce raw_write_seqcount_barrier(), a new construct that can be
used to provide write barrier semantics in seqcount read loops instead
of the usual consistency guarantee.
raw_write_seqcount_barier() is equivalent to:
raw_write_seqcount_begin();
raw_write_seqcount_end();
But avoids issueing two back-to-back smp_wmb() instructions.
This construct works because the read side will 'stall' when observing
odd values. This means that -- referring to the example in the comment
below -- even though there is no (matching) read barrier between the
loads of X and Y, we cannot observe !x && !y, because:
- if we observe Y == false we must observe the first sequence
increment, which makes us loop, until
- we observe !(seq & 1) -- the second sequence increment -- at which
time we must also observe T == true.
Suggested-by: Oleg Nesterov <o...@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: umgwanakikb...@gmail.com
Cc: ktk...@parallels.com
Cc: rost...@goodmis.org
Cc: juri.le...@gmail.com
Cc: pang.xun...@linaro.org
Cc: o...@redhat.com
Cc: wanpeng...@linux.intel.com
Cc: Al Viro <v...@zeniv.linux.org.uk>
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Link:
http://lkml.kernel.org/r/20150617122924.gp3...@twins.programming.kicks-ass.net
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
---
include/linux/seqlock.h | 41 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 41 insertions(+)
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index c07e3a5..486e685 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -233,6 +233,47 @@ static inline void raw_write_seqcount_end(seqcount_t *s)
s->sequence++;
}
+/**
+ * raw_write_seqcount_barrier - do a seq write barrier
+ * @s: pointer to seqcount_t
+ *
+ * This can be used to provide an ordering guarantee instead of the
+ * usual consistency guarantee. It is one wmb cheaper, because we can
+ * collapse the two back-to-back wmb()s.
+ *
+ * seqcount_t seq;
+ * bool X = true, Y = false;
+ *
+ * void read(void)
+ * {
+ * bool x, y;
+ *
+ * do {
+ * int s = read_seqcount_begin(&seq);
+ *
+ * x = X; y = Y;
+ *
+ * } while (read_seqcount_retry(&seq, s));
+ *
+ * BUG_ON(!x && !y);
+ * }
+ *
+ * void write(void)
+ * {
+ * Y = true;
+ *
+ * raw_write_seqcount_barrier(seq);
+ *
+ * X = false;
+ * }
+ */
+static inline void raw_write_seqcount_barrier(seqcount_t *s)
+{
+ s->sequence++;
+ smp_wmb();
+ s->sequence++;
+}
+
/*
* raw_write_seqcount_latch - redirect readers to even/odd copy
* @s: pointer to seqcount_t
--
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