[PATCH] mm/zsmalloc: fix release order of locks in zs_page_migrate()

[Richard Chang]

In zs_page_migrate(), locks are acquired in the following order:
  1. write_lock(&pool->lock)
  2. spin_lock(&class->lock)
  3. zspage_write_trylock(zspage)

However, upon successful page migration, they were being released in
forward acquisition (FIFO) order:
  1. write_unlock(&pool->lock)
  2. spin_unlock(&class->lock)
  3. zspage_write_unlock(zspage)

Fix the unlocking order to release locks in strict reverse (LIFO)
order of acquisition:
  3. zspage_write_unlock(zspage)
  2. spin_unlock(&class->lock)
  1. write_unlock(&pool->lock)

Releasing locks in reverse order of acquisition adheres to standard
kernel locking hygiene, prevents potential lock ordering and lockdep
inconsistencies.

Signed-off-by: Richard Chang <richardycc@xxxxxxxxxx>
---
 mm/zsmalloc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 83f5820c45f9..b09299115886 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1772,9 +1772,9 @@ static int zs_page_migrate(struct page *newpage, struct page *page,
 	 * Since we complete the data copy and set up new zspage structure,
 	 * it's okay to release migration_lock.
 	 */
-	write_unlock(&pool->lock);
-	spin_unlock(&class->lock);
 	zspage_write_unlock(zspage);
+	spin_unlock(&class->lock);
+	write_unlock(&pool->lock);
 
 	zpdesc_get(newzpdesc);
 	if (zpdesc_zone(newzpdesc) != zpdesc_zone(zpdesc)) {
-- 
2.55.0.229.g6434b31f56-goog