[PATCH V2] drivers: core: Remove glue dirs early only when refcount is 1

[Prateek Sood]

While loading firmware blobs parallely in different threads, it is possible
to free sysfs node of glue_dirs in device_del() from a thread while another
thread is trying to add subdir from device_add() in glue_dirs sysfs node.

    CPU1                                           CPU2
fw_load_sysfs_fallback()
  device_add()
    get_device_parent()
      class_dir_create_and_add()
        kobject_add_internal()
          create_dir() // glue_dir

                                           fw_load_sysfs_fallback()
                                             device_add()
                                               get_device_parent()
                                                 kobject_get() //glue_dir

  device_del()
    cleanup_glue_dir()
      kobject_del()

                                               kobject_add()
                                                 kobject_add_internal()
                                                   create_dir() // in glue_dir
                                                     kernfs_create_dir_ns()

       sysfs_remove_dir() //glue_dir->sd=NULL
       sysfs_put() // free glue_dir->sd

                                                       kernfs_new_node()
                                                         kernfs_get(glue_dir)

Fix this race by making sure that kernfs_node for glue_dir is released only
when refcount for glue_dir kobj is 1.

Signed-off-by: Prateek Sood <prsood@xxxxxxxxxxxxxx>
---
 drivers/base/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/base/core.c b/drivers/base/core.c
index 4aeaa0c..3955d07 100644
--- a/drivers/base/core.c
+++ b/drivers/base/core.c
@@ -1820,12 +1820,15 @@ static inline struct kobject *get_glue_dir(struct device *dev)
  */
 static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
 {
+	unsigned int refcount;
+
 	/* see if we live in a "glue" directory */
 	if (!live_in_glue_dir(glue_dir, dev))
 		return;
 
 	mutex_lock(&gdp_mutex);
-	if (!kobject_has_children(glue_dir))
+	refcount = kref_read(&glue_dir->kref);
+	if (!kobject_has_children(glue_dir) && !--refcount)
 		kobject_del(glue_dir);
 	kobject_put(glue_dir);
 	mutex_unlock(&gdp_mutex);
-- 
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