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>
diff --git a/include/linux/fdtable.h b/include/linux/fdtable.h
index 085197b..70e8e21 100644
--- a/include/linux/fdtable.h
+++ b/include/linux/fdtable.h
@@ -59,29 +59,36 @@
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);
}
/*