From a8d4b8345e0ee48b732126d980efaf0dc373e2b0 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Sat, 11 Jan 2014 19:19:32 +0100 Subject: 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 Signed-off-by: Al Viro --- fs/file.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'fs') 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); -- cgit v1.2.3