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

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rcu_dereference_check_fdtable() looks very wrong,

1. rcu_my_thread_group_empty() was added by 844b9a8707 "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 <[email protected]>
Signed-off-by: Al Viro <[email protected]>
This commit is contained in:
Oleg Nesterov 2014-01-11 19:19:32 +01:00 • committed by Al Viro
commit a8d4b8345e
4 changed files with 23 additions and 29 deletions
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4
fs/file.c
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@ -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);