UPSTREAM: 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 ]
BUG=chromium:324025
TEST=build/boot 3.8 on link
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>
(cherry picked from commit 9b0fd802e8c0545148324916055e7b40d97963fa)
Signed-off-by: Sonny Rao <sonnyrao@chromium.org>
Change-Id: I1f8bfd65661df5d556a53cc278aa2a6730c329dd
Reviewed-on: https://chromium-review.googlesource.com/300783
Commit-Ready: Sonny Rao <sonnyrao@chromium.org>
Tested-by: Sonny Rao <sonnyrao@chromium.org>
Reviewed-by: Olof Johansson <olofj@chromium.org>
diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 4793ed5..b4187b1 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -240,6 +240,17 @@
/*
+ * 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.
*/
