[PATCH] mm/shrinker: do not hold RCU lock in shrinker_debugfs_count_show()

[Shakeel Butt]

Reading the debugfs "count" file of a memcg-aware shrinker can sleep
inside an RCU read-side critical section:

  BUG: sleeping function called from invalid context at kernel/cgroup/rstat.c:421
  RCU nest depth: 1, expected: 0
   css_rstat_flush
   mem_cgroup_flush_stats
   zswap_shrinker_count
   shrinker_debugfs_count_show

shrinker_debugfs_count_show() invokes the ->count_objects() callback
under rcu_read_lock(). The zswap callback flushes memcg stats via
css_rstat_flush(), which may sleep, so it must not run under RCU.

The RCU lock is not needed here. mem_cgroup_iter() takes RCU internally
and returns a memcg holding a css reference (dropped on the next
iteration or by mem_cgroup_iter_break()), so the memcg stays alive
without it. The shrinker is kept alive by the open debugfs file:
shrinker_free() removes the debugfs entries via
debugfs_remove_recursive(), which waits for in-flight readers to drain,
before call_rcu(..., shrinker_free_rcu_cb). The sibling "scan" handler
already invokes the sleeping ->scan_objects() callback with no RCU
section.

Drop the rcu_read_lock()/rcu_read_unlock().

Fixes: 5035ebc644ae ("mm: shrinkers: introduce debugfs interface for memory shrinkers")
Reported-by: Zenghui Yu <zenghui.yu@xxxxxxxxx>
Closes: https://lore.kernel.org/all/c052a064-cddb-494f-a0d8-f8a10b4b1c4d@xxxxxxxxx/
Suggested-by: Nhat Pham <nphamcs@xxxxxxxxx>
Signed-off-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
---
 mm/shrinker_debug.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/mm/shrinker_debug.c b/mm/shrinker_debug.c
index affa64437302..cda4e86428c8 100644
--- a/mm/shrinker_debug.c
+++ b/mm/shrinker_debug.c
@@ -57,8 +57,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
 	if (!count_per_node)
 		return -ENOMEM;
 
-	rcu_read_lock();
-
 	memcg_aware = shrinker->flags & SHRINKER_MEMCG_AWARE;
 
 	memcg = mem_cgroup_iter(NULL, NULL, NULL);
@@ -88,8 +86,6 @@ static int shrinker_debugfs_count_show(struct seq_file *m, void *v)
 		}
 	} while ((memcg = mem_cgroup_iter(NULL, memcg, NULL)) != NULL);
 
-	rcu_read_unlock();
-
 	kfree(count_per_node);
 	return ret;
 }
-- 
2.53.0-Meta