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 --- include/linux/fdtable.h | 35 +++++++++++++++++++++-------------- 1 file changed, 21 insertions(+), 14 deletions(-) (limited to 'include/linux/fdtable.h') diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index 085197bd8812..70e8e21c0a30 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -59,29 +59,36 @@ struct files_struct { struct file __rcu * fd_array[NR_OPEN_DEFAULT]; }; -#define rcu_dereference_check_fdtable(files, fdtfd) \ - (rcu_dereference_check((fdtfd), \ - lockdep_is_held(&(files)->file_lock) || \ - atomic_read(&(files)->count) == 1 || \ - rcu_my_thread_group_empty())) - -#define files_fdtable(files) \ - (rcu_dereference_check_fdtable((files), (files)->fdt)) - struct file_operations; struct vfsmount; struct dentry; extern void __init files_defer_init(void); -static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd) +#define rcu_dereference_check_fdtable(files, fdtfd) \ + rcu_dereference_check((fdtfd), lockdep_is_held(&(files)->file_lock)) + +#define files_fdtable(files) \ + rcu_dereference_check_fdtable((files), (files)->fdt) + +/* + * The caller must ensure that fd table isn't shared or hold rcu or file lock + */ +static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd) { - struct file * file = NULL; - struct fdtable *fdt = files_fdtable(files); + struct fdtable *fdt = rcu_dereference_raw(files->fdt); if (fd < fdt->max_fds) - file = rcu_dereference_check_fdtable(files, fdt->fd[fd]); - return file; + return rcu_dereference_raw(fdt->fd[fd]); + return NULL; +} + +static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd) +{ + rcu_lockdep_assert(rcu_read_lock_held() || + lockdep_is_held(&files->file_lock), + "suspicious rcu_dereference_check() usage"); + return __fcheck_files(files, fd); } /* -- cgit v1.2.3