introduce __fcheck_files() to fix rcu_dereference_check_fdtable(), kill rcu_my_thread_group_empty()

rcu_dereference_check_fdtable() looks very wrong,

1. rcu_my_thread_group_empty() was added by 844b9a8707f1 "vfs: fix
   RCU-lockdep false positive due to /proc" but it doesn't really
   fix the problem. A CLONE_THREAD (without CLONE_FILES) task can
   hit the same race with get_files_struct().

   And otoh rcu_my_thread_group_empty() can suppress the correct
   warning if the caller is the CLONE_FILES (without CLONE_THREAD)
   task.

2. files->count == 1 check is not really right too. Even if this
   files_struct is not shared it is not safe to access it lockless
   unless the caller is the owner.

   Otoh, this check is sub-optimal. files->count == 0 always means
   it is safe to use it lockless even if files != current->files,
   but put_files_struct() has to take rcu_read_lock(). See the next
   patch.

This patch removes the buggy checks and turns fcheck_files() into
__fcheck_files() which uses rcu_dereference_raw(), the "unshared"
callers, fget_light() and fget_raw_light(), can use it to avoid
the warning from RCU-lockdep.

fcheck_files() is trivially reimplemented as rcu_lockdep_assert()
plus __fcheck_files().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 2 insertions, 2 deletions

diff --git a/fs/file.c b/fs/file.c
index 4a78f981557a..957cbc09b0db 100644
--- a/fs/file.c
+++ b/fs/file.c
@@ -707,7 +707,7 @@ struct file *fget_light(unsigned int fd, int *fput_needed)
 
         *fput_needed = 0;
         if (atomic_read(&files->count) == 1) {
-                file = fcheck_files(files, fd);
+                file = __fcheck_files(files, fd);
                 if (file && (file->f_mode & FMODE_PATH))
                         file = NULL;
         } else {
@@ -735,7 +735,7 @@ struct file *fget_raw_light(unsigned int fd, int *fput_needed)
 
         *fput_needed = 0;
         if (atomic_read(&files->count) == 1) {
-                file = fcheck_files(files, fd);
+                file = __fcheck_files(files, fd);
         } else {
                 rcu_read_lock();
                 file = fcheck_files(files, fd);