mm: shrinkers: fix deadlock in shrinker debugfs

The debugfs_remove_recursive() is invoked by unregister_shrinker(), which
is holding the write lock of shrinker_rwsem.  It will waits for the
handler of debugfs file complete.  The handler also needs to hold the read
lock of shrinker_rwsem to do something.  So it may cause the following
deadlock:

 	CPU0				CPU1

debugfs_file_get()
shrinker_debugfs_count_show()/shrinker_debugfs_scan_write()

     				unregister_shrinker()
				--> down_write(&shrinker_rwsem);
				    debugfs_remove_recursive()
					// wait for (A)
				    --> wait_for_completion();

    // wait for (B)
--> down_read_killable(&shrinker_rwsem)
debugfs_file_put() -- (A)

				    up_write() -- (B)

The down_read_killable() can be killed, so that the above deadlock can be
recovered.  But it still requires an extra kill action, otherwise it will
block all subsequent shrinker-related operations, so it's better to fix
it.

[[email protected]: fix CONFIG_SHRINKER_DEBUG=n stub]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 5035ebc ("mm: shrinkers: introduce debugfs interface for memory shrinkers")
Signed-off-by: Qi Zheng <[email protected]>
Reviewed-by: Roman Gushchin <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Qi Zheng

include/linux/shrinker.h

@@ -107,16 +107,17 @@ extern void synchronize_shrinkers(void);
 
 #ifdef CONFIG_SHRINKER_DEBUG
 extern int shrinker_debugfs_add(struct shrinker *shrinker);
-extern void shrinker_debugfs_remove(struct shrinker *shrinker);
+extern struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker);
 extern int __printf(2, 3) shrinker_debugfs_rename(struct shrinker *shrinker,
 						  const char *fmt, ...);
 #else /* CONFIG_SHRINKER_DEBUG */
 static inline int shrinker_debugfs_add(struct shrinker *shrinker)
 {
 	return 0;
 }
-static inline void shrinker_debugfs_remove(struct shrinker *shrinker)
+static inline struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
 {
+	return NULL;
 }
 static inline __printf(2, 3)
 int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)

mm/shrinker_debug.c

@@ -246,18 +246,21 @@ int shrinker_debugfs_rename(struct shrinker *shrinker, const char *fmt, ...)
 }
 EXPORT_SYMBOL(shrinker_debugfs_rename);
 
-void shrinker_debugfs_remove(struct shrinker *shrinker)
+struct dentry *shrinker_debugfs_remove(struct shrinker *shrinker)
 {
+	struct dentry *entry = shrinker->debugfs_entry;
+
 	lockdep_assert_held(&shrinker_rwsem);
 
 	kfree_const(shrinker->name);
 	shrinker->name = NULL;
 
-	if (!shrinker->debugfs_entry)
-		return;
+	if (entry) {
+		ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id);
+		shrinker->debugfs_entry = NULL;
+	}
 
-	debugfs_remove_recursive(shrinker->debugfs_entry);
-	ida_free(&shrinker_debugfs_ida, shrinker->debugfs_id);
+	return entry;
 }
 
 static int __init shrinker_debugfs_init(void)

mm/vmscan.c

@@ -741,6 +741,8 @@ EXPORT_SYMBOL(register_shrinker);
  */
 void unregister_shrinker(struct shrinker *shrinker)
 {
+	struct dentry *debugfs_entry;
+
 	if (!(shrinker->flags & SHRINKER_REGISTERED))
 		return;
 
@@ -749,9 +751,11 @@ void unregister_shrinker(struct shrinker *shrinker)
 	shrinker->flags &= ~SHRINKER_REGISTERED;
 	if (shrinker->flags & SHRINKER_MEMCG_AWARE)
 		unregister_memcg_shrinker(shrinker);
-	shrinker_debugfs_remove(shrinker);
+	debugfs_entry = shrinker_debugfs_remove(shrinker);
 	up_write(&shrinker_rwsem);
 
+	debugfs_remove_recursive(debugfs_entry);
+
 	kfree(shrinker->nr_deferred);
 	shrinker->nr_deferred = NULL;
 }