[4.19-stable PATCH] dax: Avoid losing wakeup in dax_lock_mapping_entry

[Dan Williams <dan.j.williams@xxxxxxxxx>]

From: Matthew Wilcox <willy@xxxxxxxxxxxxx>

commit 25bbe21bf427a81b8e3ccd480ea0e1d940256156 upstream.

After calling get_unlocked_entry(), you have to call
put_unlocked_entry() to avoid subsequent waiters losing wakeups.

Fixes: c2a7d2a11552 ("filesystem-dax: Introduce dax_lock_mapping_entry()")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
---
Passes the nvdimm unit test suite which exercises the lock path.

 fs/dax.c |    7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/dax.c b/fs/dax.c
index 0fb270f0a0ef..b0cd1364c68f 100644
--- a/fs/dax.c
+++ b/fs/dax.c
@@ -217,6 +217,9 @@ static inline void *unlock_slot(struct address_space *mapping, void **slot)
 	return (void *)entry;
 }
 
+static void put_unlocked_mapping_entry(struct address_space *mapping,
+				       pgoff_t index, void *entry);
+
 /*
  * Lookup entry in radix tree, wait for it to become unlocked if it is
  * exceptional entry and return it. The caller must call
@@ -256,8 +259,10 @@ static void *__get_unlocked_mapping_entry(struct address_space *mapping,
 		revalidate = wait_fn();
 		finish_wait(wq, &ewait.wait);
 		xa_lock_irq(&mapping->i_pages);
-		if (revalidate)
+		if (revalidate) {
+			put_unlocked_mapping_entry(mapping, index, entry);
 			return ERR_PTR(-EAGAIN);
+		}
 	}
 }