[PATCH RFC 1/1] kernfs: keep kernfs node alive for __kernfs_remove()

[Konstantin Khorenko <khorenko@virtuozzo.com>]

__kernfs_remove() which is called under kernfs_mutex,
assumes nobody kills kernfs node whie it's working on it
and "get"s current kernfs node for that.

But we hit a warning in kernfs_get(): kn->counter == 0 already:
  ------------[ cut here ]------------
  WARNING: CPU: 2 PID: 63923 at fs/kernfs/dir.c:377 kernfs_get+0x2f/0x40
  ...
  Call Trace:
   [<ffffffffa7f92e67>] dump_stack+0x19/0x1b
   [<ffffffffa78987b8>] __warn+0xd8/0x100
   [<ffffffffa78988fd>] warn_slowpath_null+0x1d/0x20
   [<ffffffffa7aecaff>] kernfs_get+0x2f/0x40
   [<ffffffffa7aed233>] __kernfs_remove+0x113/0x260
   [<ffffffffa7aee201>] kernfs_remove+0x21/0x30
   [<ffffffffa7af1010>] sysfs_remove_dir+0x50/0x80
   [<ffffffffa7b9fb38>] kobject_del+0x18/0x50
   [<ffffffffa7a38a4d>] sysfs_slab_remove+0x3d/0x50
   [<ffffffffa79f1e6b>] do_kmem_cache_release+0x3b/0x70
   [<ffffffffa79f2aa1>] memcg_destroy_kmem_caches+0xb1/0xf0
   [<ffffffffa7a4ed5c>] mem_cgroup_css_free+0x4c/0x280
   [<ffffffffa79377fc>] cgroup_free_fn+0x4c/0x120
   [<ffffffffa78bc222>] process_one_work+0x182/0x440
   [<ffffffffa78bd3d6>] worker_thread+0x126/0x3c0
   [<ffffffffa78c4441>] kthread+0xd1/0xe0

This could be for example because of kernfs_notify_workfn() which
does kernfs_put(kn) out of kernfs_mutex held section,
so move kernfs_put(kn) under the mutex.

Signed-off-by: Konstantin Khorenko <khorenko@virtuozzo.com>
---
 fs/kernfs/file.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index ae948aaa4c53..ab9c7e2064cc 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -915,8 +915,8 @@ static void kernfs_notify_workfn(struct work_struct *work)
 		iput(inode);
 	}
 
-	mutex_unlock(&kernfs_mutex);
 	kernfs_put(kn);
+	mutex_unlock(&kernfs_mutex);
 	goto repeat;
 }
 
-- 
2.15.1