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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1		Locking scheme used for directory operations is based on two
2	kinds of locks - per-inode (->i_mutex) and per-filesystem
3	(->s_vfs_rename_mutex).
4	
5		When taking the i_mutex on multiple non-directory objects, we
6	always acquire the locks in order by increasing address.  We'll call
7	that "inode pointer" order in the following.
8	
9		For our purposes all operations fall in 5 classes:
10	
11	1) read access.  Locking rules: caller locks directory we are accessing.
12	
13	2) object creation.  Locking rules: same as above.
14	
15	3) object removal.  Locking rules: caller locks parent, finds victim,
16	locks victim and calls the method.
17	
18	4) rename() that is _not_ cross-directory.  Locking rules: caller locks
19	the parent and finds source and target.  If target already exists, lock
20	it.  If source is a non-directory, lock it.  If that means we need to
21	lock both, lock them in inode pointer order.
22	
23	5) link creation.  Locking rules:
24		* lock parent
25		* check that source is not a directory
26		* lock source
27		* call the method.
28	
29	6) cross-directory rename.  The trickiest in the whole bunch.  Locking
30	rules:
31		* lock the filesystem
32		* lock parents in "ancestors first" order.
33		* find source and target.
34		* if old parent is equal to or is a descendent of target
35			fail with -ENOTEMPTY
36		* if new parent is equal to or is a descendent of source
37			fail with -ELOOP
38		* If target exists, lock it.  If source is a non-directory, lock
39		  it.  In case that means we need to lock both source and target,
40		  do so in inode pointer order.
41		* call the method.
42	
43	
44	The rules above obviously guarantee that all directories that are going to be
45	read, modified or removed by method will be locked by caller.
46	
47	
48	If no directory is its own ancestor, the scheme above is deadlock-free.
49	Proof:
50	
51		First of all, at any moment we have a partial ordering of the
52	objects - A < B iff A is an ancestor of B.
53	
54		That ordering can change.  However, the following is true:
55	
56	(1) if object removal or non-cross-directory rename holds lock on A and
57	    attempts to acquire lock on B, A will remain the parent of B until we
58	    acquire the lock on B.  (Proof: only cross-directory rename can change
59	    the parent of object and it would have to lock the parent).
60	
61	(2) if cross-directory rename holds the lock on filesystem, order will not
62	    change until rename acquires all locks.  (Proof: other cross-directory
63	    renames will be blocked on filesystem lock and we don't start changing
64	    the order until we had acquired all locks).
65	
66	(3) locks on non-directory objects are acquired only after locks on
67	    directory objects, and are acquired in inode pointer order.
68	    (Proof: all operations but renames take lock on at most one
69	    non-directory object, except renames, which take locks on source and
70	    target in inode pointer order in the case they are not directories.)
71	
72		Now consider the minimal deadlock.  Each process is blocked on
73	attempt to acquire some lock and already holds at least one lock.  Let's
74	consider the set of contended locks.  First of all, filesystem lock is
75	not contended, since any process blocked on it is not holding any locks.
76	Thus all processes are blocked on ->i_mutex.
77	
78		By (3), any process holding a non-directory lock can only be
79	waiting on another non-directory lock with a larger address.  Therefore
80	the process holding the "largest" such lock can always make progress, and
81	non-directory objects are not included in the set of contended locks.
82	
83		Thus link creation can't be a part of deadlock - it can't be
84	blocked on source and it means that it doesn't hold any locks.
85	
86		Any contended object is either held by cross-directory rename or
87	has a child that is also contended.  Indeed, suppose that it is held by
88	operation other than cross-directory rename.  Then the lock this operation
89	is blocked on belongs to child of that object due to (1).
90	
91		It means that one of the operations is cross-directory rename.
92	Otherwise the set of contended objects would be infinite - each of them
93	would have a contended child and we had assumed that no object is its
94	own descendent.  Moreover, there is exactly one cross-directory rename
95	(see above).
96	
97		Consider the object blocking the cross-directory rename.  One
98	of its descendents is locked by cross-directory rename (otherwise we
99	would again have an infinite set of contended objects).  But that
100	means that cross-directory rename is taking locks out of order.  Due
101	to (2) the order hadn't changed since we had acquired filesystem lock.
102	But locking rules for cross-directory rename guarantee that we do not
103	try to acquire lock on descendent before the lock on ancestor.
104	Contradiction.  I.e.  deadlock is impossible.  Q.E.D.
105	
106	
107		These operations are guaranteed to avoid loop creation.  Indeed,
108	the only operation that could introduce loops is cross-directory rename.
109	Since the only new (parent, child) pair added by rename() is (new parent,
110	source), such loop would have to contain these objects and the rest of it
111	would have to exist before rename().  I.e. at the moment of loop creation
112	rename() responsible for that would be holding filesystem lock and new parent
113	would have to be equal to or a descendent of source.  But that means that
114	new parent had been equal to or a descendent of source since the moment when
115	we had acquired filesystem lock and rename() would fail with -ELOOP in that
116	case.
117	
118		While this locking scheme works for arbitrary DAGs, it relies on
119	ability to check that directory is a descendent of another object.  Current
120	implementation assumes that directory graph is a tree.  This assumption is
121	also preserved by all operations (cross-directory rename on a tree that would
122	not introduce a cycle will leave it a tree and link() fails for directories).
123	
124		Notice that "directory" in the above == "anything that might have
125	children", so if we are going to introduce hybrid objects we will need
126	either to make sure that link(2) doesn't work for them or to make changes
127	in is_subdir() that would make it work even in presence of such beasts.
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