1.. SPDX-License-Identifier: GPL-2.0
2
3===================================
4File management in the Linux kernel
5===================================
6
7This document describes how locking for files (struct file)
8and file descriptor table (struct files) works.
9
10Up until 2.6.12, the file descriptor table has been protected
11with a lock (files->file_lock) and reference count (files->count).
12->file_lock protected accesses to all the file related fields
13of the table. ->count was used for sharing the file descriptor
14table between tasks cloned with CLONE_FILES flag. Typically
15this would be the case for posix threads. As with the common
16refcounting model in the kernel, the last task doing
17a put_files_struct() frees the file descriptor (fd) table.
18The files (struct file) themselves are protected using
19reference count (->f_count).
20
21In the new lock-free model of file descriptor management,
22the reference counting is similar, but the locking is
23based on RCU. The file descriptor table contains multiple
24elements - the fd sets (open_fds and close_on_exec, the
25array of file pointers, the sizes of the sets and the array
26etc.). In order for the updates to appear atomic to
27a lock-free reader, all the elements of the file descriptor
28table are in a separate structure - struct fdtable.
29files_struct contains a pointer to struct fdtable through
30which the actual fd table is accessed. Initially the
31fdtable is embedded in files_struct itself. On a subsequent
32expansion of fdtable, a new fdtable structure is allocated
33and files->fdtab points to the new structure. The fdtable
34structure is freed with RCU and lock-free readers either
35see the old fdtable or the new fdtable making the update
36appear atomic. Here are the locking rules for
37the fdtable structure -
38
391. All references to the fdtable must be done through
40   the files_fdtable() macro::
41
42	struct fdtable *fdt;
43
44	rcu_read_lock();
45
46	fdt = files_fdtable(files);
47	....
48	if (n <= fdt->max_fds)
49		....
50	...
51	rcu_read_unlock();
52
53   files_fdtable() uses rcu_dereference() macro which takes care of
54   the memory barrier requirements for lock-free dereference.
55   The fdtable pointer must be read within the read-side
56   critical section.
57
582. Reading of the fdtable as described above must be protected
59   by rcu_read_lock()/rcu_read_unlock().
60
613. For any update to the fd table, files->file_lock must
62   be held.
63
644. To look up the file structure given an fd, a reader
65   must use either lookup_fd_rcu() or files_lookup_fd_rcu() APIs. These
66   take care of barrier requirements due to lock-free lookup.
67
68   An example::
69
70	struct file *file;
71
72	rcu_read_lock();
73	file = lookup_fd_rcu(fd);
74	if (file) {
75		...
76	}
77	....
78	rcu_read_unlock();
79
805. Handling of the file structures is special. Since the look-up
81   of the fd (fget()/fget_light()) are lock-free, it is possible
82   that look-up may race with the last put() operation on the
83   file structure. This is avoided using atomic_long_inc_not_zero()
84   on ->f_count::
85
86	rcu_read_lock();
87	file = files_lookup_fd_rcu(files, fd);
88	if (file) {
89		if (atomic_long_inc_not_zero(&file->f_count))
90			*fput_needed = 1;
91		else
92		/* Didn't get the reference, someone's freed */
93			file = NULL;
94	}
95	rcu_read_unlock();
96	....
97	return file;
98
99   atomic_long_inc_not_zero() detects if refcounts is already zero or
100   goes to zero during increment. If it does, we fail
101   fget()/fget_light().
102
1036. Since both fdtable and file structures can be looked up
104   lock-free, they must be installed using rcu_assign_pointer()
105   API. If they are looked up lock-free, rcu_dereference()
106   must be used. However it is advisable to use files_fdtable()
107   and lookup_fd_rcu()/files_lookup_fd_rcu() which take care of these issues.
108
1097. While updating, the fdtable pointer must be looked up while
110   holding files->file_lock. If ->file_lock is dropped, then
111   another thread expand the files thereby creating a new
112   fdtable and making the earlier fdtable pointer stale.
113
114   For example::
115
116	spin_lock(&files->file_lock);
117	fd = locate_fd(files, file, start);
118	if (fd >= 0) {
119		/* locate_fd() may have expanded fdtable, load the ptr */
120		fdt = files_fdtable(files);
121		__set_open_fd(fd, fdt);
122		__clear_close_on_exec(fd, fdt);
123		spin_unlock(&files->file_lock);
124	.....
125
126   Since locate_fd() can drop ->file_lock (and reacquire ->file_lock),
127   the fdtable pointer (fdt) must be loaded after locate_fd().
128
129