Improvement for [md PATCH 15/15] MD: use per-cpu counter for writes_pending

[NeilBrown <neilb@xxxxxxxx>]

__ref_is_percpu() is documented as an internal interface,
so best not to use it.
We don't really need it, as ->sync_checkers is always 0
when the writes_pending is in per-cpu mode.

So change to test ->sync_checkers, and update comments to match.

Signed-off-by: NeilBrown <neilb@xxxxxxxx>
---
 drivers/md/md.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index adf2b5bdfd67..b76ac563115e 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -2266,8 +2266,8 @@ static bool set_in_sync(struct mddev *mddev)
 		    percpu_ref_is_zero(&mddev->writes_pending)) {
 			mddev->in_sync = 1;
 			/*
-			 * Ensure in_sync is visible before switch back
-			 * to percpu
+			 * Ensure ->in_sync is visible before we clear
+			 * ->sync_checkers.
 			 */
 			smp_mb();
 			set_bit(MD_SB_CHANGE_CLEAN, &mddev->sb_flags);
@@ -7920,7 +7920,7 @@ void md_write_start(struct mddev *mddev, struct bio *bi)
 	smp_mb(); /* Match smp_mb in set_in_sync() */
 	if (mddev->safemode == 1)
 		mddev->safemode = 0;
-	if (mddev->in_sync || !__ref_is_percpu(&mddev->writes_pending, &notused)) {
+	if (mddev->in_sync || !mddev->sync_checkers) {
 		spin_lock(&mddev->lock);
 		if (mddev->in_sync) {
 			mddev->in_sync = 0;
-- 
2.12.0

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