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1 Locking scheme used for directory operations is based on two
2kinds of locks - per-inode (->i_mutex) and per-filesystem
3(->s_vfs_rename_mutex).
4
5 For our purposes all operations fall in 5 classes:
6
71) read access. Locking rules: caller locks directory we are accessing.
8
92) object creation. Locking rules: same as above.
10
113) object removal. Locking rules: caller locks parent, finds victim,
12locks victim and calls the method.
13
144) rename() that is _not_ cross-directory. Locking rules: caller locks
15the parent, finds source and target, if target already exists - locks it
16and then calls the method.
17
185) link creation. Locking rules:
19 * lock parent
20 * check that source is not a directory
21 * lock source
22 * call the method.
23
246) cross-directory rename. The trickiest in the whole bunch. Locking
25rules:
26 * lock the filesystem
27 * lock parents in "ancestors first" order.
28 * find source and target.
29 * if old parent is equal to or is a descendent of target
30 fail with -ENOTEMPTY
31 * if new parent is equal to or is a descendent of source
32 fail with -ELOOP
33 * if target exists - lock it.
34 * call the method.
35
36
37The rules above obviously guarantee that all directories that are going to be
38read, modified or removed by method will be locked by caller.
39
40
41If no directory is its own ancestor, the scheme above is deadlock-free.
42Proof:
43
44 First of all, at any moment we have a partial ordering of the
45objects - A < B iff A is an ancestor of B.
46
47 That ordering can change. However, the following is true:
48
49(1) if object removal or non-cross-directory rename holds lock on A and
50 attempts to acquire lock on B, A will remain the parent of B until we
51 acquire the lock on B. (Proof: only cross-directory rename can change
52 the parent of object and it would have to lock the parent).
53
54(2) if cross-directory rename holds the lock on filesystem, order will not
55 change until rename acquires all locks. (Proof: other cross-directory
56 renames will be blocked on filesystem lock and we don't start changing
57 the order until we had acquired all locks).
58
59(3) any operation holds at most one lock on non-directory object and
60 that lock is acquired after all other locks. (Proof: see descriptions
61 of operations).
62
63 Now consider the minimal deadlock. Each process is blocked on
64attempt to acquire some lock and already holds at least one lock. Let's
65consider the set of contended locks. First of all, filesystem lock is
66not contended, since any process blocked on it is not holding any locks.
67Thus all processes are blocked on ->i_mutex.
68
69 Non-directory objects are not contended due to (3). Thus link
70creation can't be a part of deadlock - it can't be blocked on source
71and it means that it doesn't hold any locks.
72
73 Any contended object is either held by cross-directory rename or
74has a child that is also contended. Indeed, suppose that it is held by
75operation other than cross-directory rename. Then the lock this operation
76is blocked on belongs to child of that object due to (1).
77
78 It means that one of the operations is cross-directory rename.
79Otherwise the set of contended objects would be infinite - each of them
80would have a contended child and we had assumed that no object is its
81own descendent. Moreover, there is exactly one cross-directory rename
82(see above).
83
84 Consider the object blocking the cross-directory rename. One
85of its descendents is locked by cross-directory rename (otherwise we
86would again have an infinite set of contended objects). But that
87means that cross-directory rename is taking locks out of order. Due
88to (2) the order hadn't changed since we had acquired filesystem lock.
89But locking rules for cross-directory rename guarantee that we do not
90try to acquire lock on descendent before the lock on ancestor.
91Contradiction. I.e. deadlock is impossible. Q.E.D.
92
93
94 These operations are guaranteed to avoid loop creation. Indeed,
95the only operation that could introduce loops is cross-directory rename.
96Since the only new (parent, child) pair added by rename() is (new parent,
97source), such loop would have to contain these objects and the rest of it
98would have to exist before rename(). I.e. at the moment of loop creation
99rename() responsible for that would be holding filesystem lock and new parent
100would have to be equal to or a descendent of source. But that means that
101new parent had been equal to or a descendent of source since the moment when
102we had acquired filesystem lock and rename() would fail with -ELOOP in that
103case.
104
105 While this locking scheme works for arbitrary DAGs, it relies on
106ability to check that directory is a descendent of another object. Current
107implementation assumes that directory graph is a tree. This assumption is
108also preserved by all operations (cross-directory rename on a tree that would
109not introduce a cycle will leave it a tree and link() fails for directories).
110
111 Notice that "directory" in the above == "anything that might have
112children", so if we are going to introduce hybrid objects we will need
113either to make sure that link(2) doesn't work for them or to make changes
114in is_subdir() that would make it work even in presence of such beasts.
1 Locking scheme used for directory operations is based on two
2kinds of locks - per-inode (->i_mutex) and per-filesystem
3(->s_vfs_rename_mutex).
4
5 For our purposes all operations fall in 5 classes:
6
71) read access. Locking rules: caller locks directory we are accessing.
8
92) object creation. Locking rules: same as above.
10
113) object removal. Locking rules: caller locks parent, finds victim,
12locks victim and calls the method.
13
144) rename() that is _not_ cross-directory. Locking rules: caller locks
15the parent, finds source and target, if target already exists - locks it
16and then calls the method.
17
185) link creation. Locking rules:
19 * lock parent
20 * check that source is not a directory
21 * lock source
22 * call the method.
23
246) cross-directory rename. The trickiest in the whole bunch. Locking
25rules:
26 * lock the filesystem
27 * lock parents in "ancestors first" order.
28 * find source and target.
29 * if old parent is equal to or is a descendent of target
30 fail with -ENOTEMPTY
31 * if new parent is equal to or is a descendent of source
32 fail with -ELOOP
33 * if target exists - lock it.
34 * call the method.
35
36
37The rules above obviously guarantee that all directories that are going to be
38read, modified or removed by method will be locked by caller.
39
40
41If no directory is its own ancestor, the scheme above is deadlock-free.
42Proof:
43
44 First of all, at any moment we have a partial ordering of the
45objects - A < B iff A is an ancestor of B.
46
47 That ordering can change. However, the following is true:
48
49(1) if object removal or non-cross-directory rename holds lock on A and
50 attempts to acquire lock on B, A will remain the parent of B until we
51 acquire the lock on B. (Proof: only cross-directory rename can change
52 the parent of object and it would have to lock the parent).
53
54(2) if cross-directory rename holds the lock on filesystem, order will not
55 change until rename acquires all locks. (Proof: other cross-directory
56 renames will be blocked on filesystem lock and we don't start changing
57 the order until we had acquired all locks).
58
59(3) any operation holds at most one lock on non-directory object and
60 that lock is acquired after all other locks. (Proof: see descriptions
61 of operations).
62
63 Now consider the minimal deadlock. Each process is blocked on
64attempt to acquire some lock and already holds at least one lock. Let's
65consider the set of contended locks. First of all, filesystem lock is
66not contended, since any process blocked on it is not holding any locks.
67Thus all processes are blocked on ->i_mutex.
68
69 Non-directory objects are not contended due to (3). Thus link
70creation can't be a part of deadlock - it can't be blocked on source
71and it means that it doesn't hold any locks.
72
73 Any contended object is either held by cross-directory rename or
74has a child that is also contended. Indeed, suppose that it is held by
75operation other than cross-directory rename. Then the lock this operation
76is blocked on belongs to child of that object due to (1).
77
78 It means that one of the operations is cross-directory rename.
79Otherwise the set of contended objects would be infinite - each of them
80would have a contended child and we had assumed that no object is its
81own descendent. Moreover, there is exactly one cross-directory rename
82(see above).
83
84 Consider the object blocking the cross-directory rename. One
85of its descendents is locked by cross-directory rename (otherwise we
86would again have an infinite set of contended objects). But that
87means that cross-directory rename is taking locks out of order. Due
88to (2) the order hadn't changed since we had acquired filesystem lock.
89But locking rules for cross-directory rename guarantee that we do not
90try to acquire lock on descendent before the lock on ancestor.
91Contradiction. I.e. deadlock is impossible. Q.E.D.
92
93
94 These operations are guaranteed to avoid loop creation. Indeed,
95the only operation that could introduce loops is cross-directory rename.
96Since the only new (parent, child) pair added by rename() is (new parent,
97source), such loop would have to contain these objects and the rest of it
98would have to exist before rename(). I.e. at the moment of loop creation
99rename() responsible for that would be holding filesystem lock and new parent
100would have to be equal to or a descendent of source. But that means that
101new parent had been equal to or a descendent of source since the moment when
102we had acquired filesystem lock and rename() would fail with -ELOOP in that
103case.
104
105 While this locking scheme works for arbitrary DAGs, it relies on
106ability to check that directory is a descendent of another object. Current
107implementation assumes that directory graph is a tree. This assumption is
108also preserved by all operations (cross-directory rename on a tree that would
109not introduce a cycle will leave it a tree and link() fails for directories).
110
111 Notice that "directory" in the above == "anything that might have
112children", so if we are going to introduce hybrid objects we will need
113either to make sure that link(2) doesn't work for them or to make changes
114in is_subdir() that would make it work even in presence of such beasts.