[PATCH] md: fix refcount problem on mddev when stopping array.

[NeilBrown <neilb@suse.com>]

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md_open() gets a counted reference on an mddev using mddev_find().
If it ends up returning an error, it must drop this reference.

There are two error paths where the reference is not dropped.
One only happens if the process is signalled and an awkward time,
which is quite unlikely.
The other was introduced recently in commit af8d8e6f0.

Change the code to ensure the drop the reference when returning an error,
and make it harded to re-introduce this sort of bug in the future.

Reported-by: Marc Smith <marc.smith@mcc.edu>
Fixes: af8d8e6f0315 ("md: changes for MD_STILL_CLOSED flag")
Signed-off-by: NeilBrown <neilb@suse.com>
---

Hi Shaohua,
 as this bug was introduced in v4.9-rc1, it would be great if this
 patch could get to Linus before v4.9-final comes out (on Sunday?).

Thanks,
NeilBrown


 drivers/md/md.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 2089d46b0eb8..d1a291ac2a75 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7092,7 +7092,8 @@ static int md_open(struct block_device *bdev, fmode_t mode)
 
 	if (test_bit(MD_CLOSING, &mddev->flags)) {
 		mutex_unlock(&mddev->open_mutex);
-		return -ENODEV;
+		err = -ENODEV;
+		goto out;
 	}
 
 	err = 0;
@@ -7101,6 +7102,8 @@ static int md_open(struct block_device *bdev, fmode_t mode)
 
 	check_disk_change(bdev);
  out:
+	if (err)
+		mddev_put(mddev);
 	return err;
 }
 
-- 
2.10.2


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