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1da177e4 LT |
1 | Locking scheme used for directory operations is based on two |
2 | kinds of locks - per-inode (->i_sem) and per-filesystem (->s_vfs_rename_sem). | |
3 | ||
4 | For our purposes all operations fall in 5 classes: | |
5 | ||
6 | 1) read access. Locking rules: caller locks directory we are accessing. | |
7 | ||
8 | 2) object creation. Locking rules: same as above. | |
9 | ||
10 | 3) object removal. Locking rules: caller locks parent, finds victim, | |
11 | locks victim and calls the method. | |
12 | ||
13 | 4) rename() that is _not_ cross-directory. Locking rules: caller locks | |
14 | the parent, finds source and target, if target already exists - locks it | |
15 | and then calls the method. | |
16 | ||
17 | 5) link creation. Locking rules: | |
18 | * lock parent | |
19 | * check that source is not a directory | |
20 | * lock source | |
21 | * call the method. | |
22 | ||
23 | 6) cross-directory rename. The trickiest in the whole bunch. Locking | |
24 | rules: | |
25 | * lock the filesystem | |
26 | * lock parents in "ancestors first" order. | |
27 | * find source and target. | |
28 | * if old parent is equal to or is a descendent of target | |
29 | fail with -ENOTEMPTY | |
30 | * if new parent is equal to or is a descendent of source | |
31 | fail with -ELOOP | |
32 | * if target exists - lock it. | |
33 | * call the method. | |
34 | ||
35 | ||
36 | The rules above obviously guarantee that all directories that are going to be | |
37 | read, modified or removed by method will be locked by caller. | |
38 | ||
39 | ||
40 | If no directory is its own ancestor, the scheme above is deadlock-free. | |
41 | Proof: | |
42 | ||
43 | First of all, at any moment we have a partial ordering of the | |
44 | objects - A < B iff A is an ancestor of B. | |
45 | ||
46 | That ordering can change. However, the following is true: | |
47 | ||
48 | (1) if object removal or non-cross-directory rename holds lock on A and | |
49 | attempts to acquire lock on B, A will remain the parent of B until we | |
50 | acquire the lock on B. (Proof: only cross-directory rename can change | |
51 | the parent of object and it would have to lock the parent). | |
52 | ||
53 | (2) if cross-directory rename holds the lock on filesystem, order will not | |
54 | change until rename acquires all locks. (Proof: other cross-directory | |
55 | renames will be blocked on filesystem lock and we don't start changing | |
56 | the order until we had acquired all locks). | |
57 | ||
58 | (3) any operation holds at most one lock on non-directory object and | |
59 | that lock is acquired after all other locks. (Proof: see descriptions | |
60 | of operations). | |
61 | ||
62 | Now consider the minimal deadlock. Each process is blocked on | |
63 | attempt to acquire some lock and already holds at least one lock. Let's | |
64 | consider the set of contended locks. First of all, filesystem lock is | |
65 | not contended, since any process blocked on it is not holding any locks. | |
66 | Thus all processes are blocked on ->i_sem. | |
67 | ||
68 | Non-directory objects are not contended due to (3). Thus link | |
69 | creation can't be a part of deadlock - it can't be blocked on source | |
70 | and it means that it doesn't hold any locks. | |
71 | ||
72 | Any contended object is either held by cross-directory rename or | |
73 | has a child that is also contended. Indeed, suppose that it is held by | |
74 | operation other than cross-directory rename. Then the lock this operation | |
75 | is blocked on belongs to child of that object due to (1). | |
76 | ||
77 | It means that one of the operations is cross-directory rename. | |
78 | Otherwise the set of contended objects would be infinite - each of them | |
79 | would have a contended child and we had assumed that no object is its | |
80 | own descendent. Moreover, there is exactly one cross-directory rename | |
81 | (see above). | |
82 | ||
83 | Consider the object blocking the cross-directory rename. One | |
84 | of its descendents is locked by cross-directory rename (otherwise we | |
670e9f34 | 85 | would again have an infinite set of contended objects). But that |
1da177e4 LT |
86 | means that cross-directory rename is taking locks out of order. Due |
87 | to (2) the order hadn't changed since we had acquired filesystem lock. | |
88 | But locking rules for cross-directory rename guarantee that we do not | |
89 | try to acquire lock on descendent before the lock on ancestor. | |
90 | Contradiction. I.e. deadlock is impossible. Q.E.D. | |
91 | ||
92 | ||
93 | These operations are guaranteed to avoid loop creation. Indeed, | |
94 | the only operation that could introduce loops is cross-directory rename. | |
95 | Since the only new (parent, child) pair added by rename() is (new parent, | |
96 | source), such loop would have to contain these objects and the rest of it | |
97 | would have to exist before rename(). I.e. at the moment of loop creation | |
98 | rename() responsible for that would be holding filesystem lock and new parent | |
99 | would have to be equal to or a descendent of source. But that means that | |
100 | new parent had been equal to or a descendent of source since the moment when | |
101 | we had acquired filesystem lock and rename() would fail with -ELOOP in that | |
102 | case. | |
103 | ||
104 | While this locking scheme works for arbitrary DAGs, it relies on | |
105 | ability to check that directory is a descendent of another object. Current | |
106 | implementation assumes that directory graph is a tree. This assumption is | |
107 | also preserved by all operations (cross-directory rename on a tree that would | |
108 | not introduce a cycle will leave it a tree and link() fails for directories). | |
109 | ||
110 | Notice that "directory" in the above == "anything that might have | |
111 | children", so if we are going to introduce hybrid objects we will need | |
112 | either to make sure that link(2) doesn't work for them or to make changes | |
113 | in is_subdir() that would make it work even in presence of such beasts. |