xref: /openbmc/linux/fs/jffs2/README.Locking (revision 7d200960)
1	$Id: README.Locking,v 1.12 2005/04/13 13:22:35 dwmw2 Exp $
2
3	JFFS2 LOCKING DOCUMENTATION
4	---------------------------
5
6At least theoretically, JFFS2 does not require the Big Kernel Lock
7(BKL), which was always helpfully obtained for it by Linux 2.4 VFS
8code. It has its own locking, as described below.
9
10This document attempts to describe the existing locking rules for
11JFFS2. It is not expected to remain perfectly up to date, but ought to
12be fairly close.
13
14
15	alloc_sem
16	---------
17
18The alloc_sem is a per-filesystem semaphore, used primarily to ensure
19contiguous allocation of space on the medium. It is automatically
20obtained during space allocations (jffs2_reserve_space()) and freed
21upon write completion (jffs2_complete_reservation()). Note that
22the garbage collector will obtain this right at the beginning of
23jffs2_garbage_collect_pass() and release it at the end, thereby
24preventing any other write activity on the file system during a
25garbage collect pass.
26
27When writing new nodes, the alloc_sem must be held until the new nodes
28have been properly linked into the data structures for the inode to
29which they belong. This is for the benefit of NAND flash - adding new
30nodes to an inode may obsolete old ones, and by holding the alloc_sem
31until this happens we ensure that any data in the write-buffer at the
32time this happens are part of the new node, not just something that
33was written afterwards. Hence, we can ensure the newly-obsoleted nodes
34don't actually get erased until the write-buffer has been flushed to
35the medium.
36
37With the introduction of NAND flash support and the write-buffer,
38the alloc_sem is also used to protect the wbuf-related members of the
39jffs2_sb_info structure. Atomically reading the wbuf_len member to see
40if the wbuf is currently holding any data is permitted, though.
41
42Ordering constraints: See f->sem.
43
44
45	File Semaphore f->sem
46	---------------------
47
48This is the JFFS2-internal equivalent of the inode semaphore i->i_sem.
49It protects the contents of the jffs2_inode_info private inode data,
50including the linked list of node fragments (but see the notes below on
51erase_completion_lock), etc.
52
53The reason that the i_sem itself isn't used for this purpose is to
54avoid deadlocks with garbage collection -- the VFS will lock the i_sem
55before calling a function which may need to allocate space. The
56allocation may trigger garbage-collection, which may need to move a
57node belonging to the inode which was locked in the first place by the
58VFS. If the garbage collection code were to attempt to lock the i_sem
59of the inode from which it's garbage-collecting a physical node, this
60lead to deadlock, unless we played games with unlocking the i_sem
61before calling the space allocation functions.
62
63Instead of playing such games, we just have an extra internal
64semaphore, which is obtained by the garbage collection code and also
65by the normal file system code _after_ allocation of space.
66
67Ordering constraints:
68
69	1. Never attempt to allocate space or lock alloc_sem with
70	   any f->sem held.
71	2. Never attempt to lock two file semaphores in one thread.
72	   No ordering rules have been made for doing so.
73
74
75	erase_completion_lock spinlock
76	------------------------------
77
78This is used to serialise access to the eraseblock lists, to the
79per-eraseblock lists of physical jffs2_raw_node_ref structures, and
80(NB) the per-inode list of physical nodes. The latter is a special
81case - see below.
82
83As the MTD API no longer permits erase-completion callback functions
84to be called from bottom-half (timer) context (on the basis that nobody
85ever actually implemented such a thing), it's now sufficient to use
86a simple spin_lock() rather than spin_lock_bh().
87
88Note that the per-inode list of physical nodes (f->nodes) is a special
89case. Any changes to _valid_ nodes (i.e. ->flash_offset & 1 == 0) in
90the list are protected by the file semaphore f->sem. But the erase
91code may remove _obsolete_ nodes from the list while holding only the
92erase_completion_lock. So you can walk the list only while holding the
93erase_completion_lock, and can drop the lock temporarily mid-walk as
94long as the pointer you're holding is to a _valid_ node, not an
95obsolete one.
96
97The erase_completion_lock is also used to protect the c->gc_task
98pointer when the garbage collection thread exits. The code to kill the
99GC thread locks it, sends the signal, then unlocks it - while the GC
100thread itself locks it, zeroes c->gc_task, then unlocks on the exit path.
101
102
103	inocache_lock spinlock
104	----------------------
105
106This spinlock protects the hashed list (c->inocache_list) of the
107in-core jffs2_inode_cache objects (each inode in JFFS2 has the
108correspondent jffs2_inode_cache object). So, the inocache_lock
109has to be locked while walking the c->inocache_list hash buckets.
110
111This spinlock also covers allocation of new inode numbers, which is
112currently just '++->highest_ino++', but might one day get more complicated
113if we need to deal with wrapping after 4 milliard inode numbers are used.
114
115Note, the f->sem guarantees that the correspondent jffs2_inode_cache
116will not be removed. So, it is allowed to access it without locking
117the inocache_lock spinlock.
118
119Ordering constraints:
120
121	If both erase_completion_lock and inocache_lock are needed, the
122	c->erase_completion has to be acquired first.
123
124
125	erase_free_sem
126	--------------
127
128This semaphore is only used by the erase code which frees obsolete
129node references and the jffs2_garbage_collect_deletion_dirent()
130function. The latter function on NAND flash must read _obsolete_ nodes
131to determine whether the 'deletion dirent' under consideration can be
132discarded or whether it is still required to show that an inode has
133been unlinked. Because reading from the flash may sleep, the
134erase_completion_lock cannot be held, so an alternative, more
135heavyweight lock was required to prevent the erase code from freeing
136the jffs2_raw_node_ref structures in question while the garbage
137collection code is looking at them.
138
139Suggestions for alternative solutions to this problem would be welcomed.
140
141
142	wbuf_sem
143	--------
144
145This read/write semaphore protects against concurrent access to the
146write-behind buffer ('wbuf') used for flash chips where we must write
147in blocks. It protects both the contents of the wbuf and the metadata
148which indicates which flash region (if any) is currently covered by
149the buffer.
150
151Ordering constraints:
152	Lock wbuf_sem last, after the alloc_sem or and f->sem.
153