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 adds the new helper which simply does rcu_lock_acquire/release around fcheck_files(). The files->count == 1 callers, fget_light() and fget_raw_light(), can use this helper to avoid the warning from RCU-lockdep. Signed-off-by: Oleg Nesterov <[email protected]> --- fs/file.c | 4 ++-- include/linux/fdtable.h | 19 +++++++++++++------ include/linux/rcupdate.h | 2 -- kernel/rcu/update.c | 11 ----------- 4 files changed, 15 insertions(+), 21 deletions(-) diff --git a/fs/file.c b/fs/file.c index 4a78f98..957cbc0 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); diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h index 085197b..f39d50a 100644 --- a/include/linux/fdtable.h +++ b/include/linux/fdtable.h @@ -60,10 +60,7 @@ struct files_struct { }; #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())) + rcu_dereference_check((fdtfd), lockdep_is_held(&(files)->file_lock)) #define files_fdtable(files) \ (rcu_dereference_check_fdtable((files), (files)->fdt)) @@ -74,9 +71,9 @@ struct dentry; extern void __init files_defer_init(void); -static inline struct file * fcheck_files(struct files_struct *files, unsigned int fd) +static inline struct file *fcheck_files(struct files_struct *files, unsigned int fd) { - struct file * file = NULL; + struct file *file = NULL; struct fdtable *fdt = files_fdtable(files); if (fd < fdt->max_fds) @@ -84,6 +81,16 @@ static inline struct file * fcheck_files(struct files_struct *files, unsigned in return file; } +/* The caller must ensure that fd table isn't shared */ +static inline struct file *__fcheck_files(struct files_struct *files, unsigned int fd) +{ + struct file *file; + /* make rcu_dereference_check_fdtable() happy */ + rcu_lock_acquire(&rcu_lock_map); + file = fcheck_files(files, fd); + rcu_lock_release(&rcu_lock_map); + return file; +} /* * Check whether the specified fd has an open file. */ diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 39cbb88..a2482cf 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -448,8 +448,6 @@ static inline int rcu_read_lock_sched_held(void) #ifdef CONFIG_PROVE_RCU -extern int rcu_my_thread_group_empty(void); - /** * rcu_lockdep_assert - emit lockdep splat if specified condition not met * @c: condition to check diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 6cb3dff..a3596c8 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -195,17 +195,6 @@ void wait_rcu_gp(call_rcu_func_t crf) } EXPORT_SYMBOL_GPL(wait_rcu_gp); -#ifdef CONFIG_PROVE_RCU -/* - * wrapper function to avoid #include problems. - */ -int rcu_my_thread_group_empty(void) -{ - return thread_group_empty(current); -} -EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty); -#endif /* #ifdef CONFIG_PROVE_RCU */ - #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD static inline void debug_init_rcu_head(struct rcu_head *head) { -- 1.5.5.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
