Based on kernel version 3.9. Page generated on 2013-05-02 23:06 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 For our purposes all operations fall in 5 classes: 6 7 1) read access. Locking rules: caller locks directory we are accessing. 8 9 2) object creation. Locking rules: same as above. 10 11 3) object removal. Locking rules: caller locks parent, finds victim, 12 locks victim and calls the method. 13 14 4) rename() that is _not_ cross-directory. Locking rules: caller locks 15 the parent, finds source and target, if target already exists - locks it 16 and then calls the method. 17 18 5) link creation. Locking rules: 19 * lock parent 20 * check that source is not a directory 21 * lock source 22 * call the method. 23 24 6) cross-directory rename. The trickiest in the whole bunch. Locking 25 rules: 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 37 The rules above obviously guarantee that all directories that are going to be 38 read, modified or removed by method will be locked by caller. 39 40 41 If no directory is its own ancestor, the scheme above is deadlock-free. 42 Proof: 43 44 First of all, at any moment we have a partial ordering of the 45 objects - 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 64 attempt to acquire some lock and already holds at least one lock. Let's 65 consider the set of contended locks. First of all, filesystem lock is 66 not contended, since any process blocked on it is not holding any locks. 67 Thus all processes are blocked on ->i_mutex. 68 69 Non-directory objects are not contended due to (3). Thus link 70 creation can't be a part of deadlock - it can't be blocked on source 71 and it means that it doesn't hold any locks. 72 73 Any contended object is either held by cross-directory rename or 74 has a child that is also contended. Indeed, suppose that it is held by 75 operation other than cross-directory rename. Then the lock this operation 76 is blocked on belongs to child of that object due to (1). 77 78 It means that one of the operations is cross-directory rename. 79 Otherwise the set of contended objects would be infinite - each of them 80 would have a contended child and we had assumed that no object is its 81 own descendent. Moreover, there is exactly one cross-directory rename 82 (see above). 83 84 Consider the object blocking the cross-directory rename. One 85 of its descendents is locked by cross-directory rename (otherwise we 86 would again have an infinite set of contended objects). But that 87 means that cross-directory rename is taking locks out of order. Due 88 to (2) the order hadn't changed since we had acquired filesystem lock. 89 But locking rules for cross-directory rename guarantee that we do not 90 try to acquire lock on descendent before the lock on ancestor. 91 Contradiction. I.e. deadlock is impossible. Q.E.D. 92 93 94 These operations are guaranteed to avoid loop creation. Indeed, 95 the only operation that could introduce loops is cross-directory rename. 96 Since the only new (parent, child) pair added by rename() is (new parent, 97 source), such loop would have to contain these objects and the rest of it 98 would have to exist before rename(). I.e. at the moment of loop creation 99 rename() responsible for that would be holding filesystem lock and new parent 100 would have to be equal to or a descendent of source. But that means that 101 new parent had been equal to or a descendent of source since the moment when 102 we had acquired filesystem lock and rename() would fail with -ELOOP in that 103 case. 104 105 While this locking scheme works for arbitrary DAGs, it relies on 106 ability to check that directory is a descendent of another object. Current 107 implementation assumes that directory graph is a tree. This assumption is 108 also preserved by all operations (cross-directory rename on a tree that would 109 not 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 112 children", so if we are going to introduce hybrid objects we will need 113 either to make sure that link(2) doesn't work for them or to make changes 114 in is_subdir() that would make it work even in presence of such beasts.