[PATCH] mm: vmscan: make unregister_shrinker() safer

[Aliaksei Karaliou <akaraliou.dev@gmail.com>]

unregister_shrinker() does not have any sanitizing inside so
calling it twice will oops because of double free attempt or so.
This patch makes unregister_shrinker() safer and allows calling
it on resource freeing path without explicit knowledge of whether
shrinker was successfully registered or not.

Signed-off-by: Aliaksei Karaliou <akaraliou.dev@gmail.com>
---
 mm/vmscan.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 65c4fa26abfa..7cb56db5e9ca 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -281,10 +281,14 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
+	if (!shrinker->nr_deferred)
+		return;
+
 	down_write(&shrinker_rwsem);
 	list_del(&shrinker->list);
 	up_write(&shrinker_rwsem);
 	kfree(shrinker->nr_deferred);
+	shrinker->nr_deferred = NULL;
 }
 EXPORT_SYMBOL(unregister_shrinker);
 
-- 
2.11.0

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