seqcount: Add raw_write_seqcount_latch()
For NMI safe access to clock monotonic we use the seqcount LSB as
index of a timekeeper array. The update sequence looks like this:
smp_wmb(); <- prior stores to a[1]
seq++;
smp_wmb(); <- seq increment before update of a[0]
update(a[0]);
smp_wmb(); <- update of a[0]
seq++;
smp_wmb(); <- seq increment before update of a[1]
update(a[1]);
To avoid open coded barriers, provide a helper function.
[ tglx: Split out of a combo patch against the first implementation of
the NMI safe accessor ]
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index dcc64b9..cce6e74 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -236,6 +236,17 @@
}
/*
+ * raw_write_seqcount_latch - redirect readers to even/odd copy
+ * @s: pointer to seqcount_t
+ */
+static inline void raw_write_seqcount_latch(seqcount_t *s)
+{
+ smp_wmb(); /* prior stores before incrementing "sequence" */
+ s->sequence++;
+ smp_wmb(); /* increment "sequence" before following stores */
+}
+
+/*
* Sequence counter only version assumes that callers are using their
* own mutexing.
*/