[PATCH RT 21/22] seqlock: provide the same ordering semantics as mainline

[Julia Cartwright]

4.9.115-rt94-rc1 stable review patch.
If you have any objection to the inclusion of this patch, let me know.

--- 8< --- 8< --- 8< ---
The mainline implementation of read_seqbegin() orders prior loads w.r.t.
the read-side critical section.  Fixup the RT writer-boosting
implementation to provide the same guarantee.

Also, while we're here, update the usage of ACCESS_ONCE() to use
READ_ONCE().

Fixes: e69f15cf77c23 ("seqlock: Prevent rt starvation")
Cc: stable-rt@xxxxxxxxxxxxxxx
Signed-off-by: Julia Cartwright <julia@xxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
(cherry picked from commit afa4c06b89a3c0fb7784ff900ccd707bef519cb7)
Signed-off-by: Julia Cartwright <julia@xxxxxx>
---
 include/linux/seqlock.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 3d7223ffdd3b..de04d6d0face 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -461,6 +461,7 @@ static inline unsigned read_seqbegin(seqlock_t *sl)
 		spin_unlock_wait(&sl->lock);
 		goto repeat;
 	}
+	smp_rmb();
 	return ret;
 }
 #endif
-- 
2.18.0