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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 <oleg@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Oleg Nesterov
2014-01-11 19:19:32 +01:00
committed by Al Viro
parent 2ccdc41319
commit a8d4b8345e
4 changed files with 23 additions and 29 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/linux/fdtable.h
+21 -14
View File
@@ -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);
}
/*