146 строки
6.3 KiB
ReStructuredText
146 строки
6.3 KiB
ReStructuredText
=================
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Directory Locking
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=================
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Locking scheme used for directory operations is based on two
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kinds of locks - per-inode (->i_rwsem) and per-filesystem
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(->s_vfs_rename_mutex).
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When taking the i_rwsem on multiple non-directory objects, we
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always acquire the locks in order by increasing address. We'll call
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that "inode pointer" order in the following.
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For our purposes all operations fall in 5 classes:
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1) read access. Locking rules: caller locks directory we are accessing.
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The lock is taken shared.
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2) object creation. Locking rules: same as above, but the lock is taken
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exclusive.
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3) object removal. Locking rules: caller locks parent, finds victim,
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locks victim and calls the method. Locks are exclusive.
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4) rename() that is _not_ cross-directory. Locking rules: caller locks
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the parent and finds source and target. In case of exchange (with
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RENAME_EXCHANGE in flags argument) lock both. In any case,
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if the target already exists, lock it. If the source is a non-directory,
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lock it. If we need to lock both, lock them in inode pointer order.
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Then call the method. All locks are exclusive.
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NB: we might get away with locking the source (and target in exchange
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case) shared.
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5) link creation. Locking rules:
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* lock parent
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* check that source is not a directory
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* lock source
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* call the method.
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All locks are exclusive.
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6) cross-directory rename. The trickiest in the whole bunch. Locking
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rules:
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* lock the filesystem
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* lock parents in "ancestors first" order.
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* find source and target.
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* if old parent is equal to or is a descendent of target
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fail with -ENOTEMPTY
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* if new parent is equal to or is a descendent of source
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fail with -ELOOP
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* If it's an exchange, lock both the source and the target.
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* If the target exists, lock it. If the source is a non-directory,
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lock it. If we need to lock both, do so in inode pointer order.
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* call the method.
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All ->i_rwsem are taken exclusive. Again, we might get away with locking
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the source (and target in exchange case) shared.
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The rules above obviously guarantee that all directories that are going to be
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read, modified or removed by method will be locked by caller.
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If no directory is its own ancestor, the scheme above is deadlock-free.
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Proof:
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First of all, at any moment we have a partial ordering of the
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objects - A < B iff A is an ancestor of B.
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That ordering can change. However, the following is true:
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(1) if object removal or non-cross-directory rename holds lock on A and
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attempts to acquire lock on B, A will remain the parent of B until we
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acquire the lock on B. (Proof: only cross-directory rename can change
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the parent of object and it would have to lock the parent).
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(2) if cross-directory rename holds the lock on filesystem, order will not
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change until rename acquires all locks. (Proof: other cross-directory
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renames will be blocked on filesystem lock and we don't start changing
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the order until we had acquired all locks).
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(3) locks on non-directory objects are acquired only after locks on
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directory objects, and are acquired in inode pointer order.
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(Proof: all operations but renames take lock on at most one
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non-directory object, except renames, which take locks on source and
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target in inode pointer order in the case they are not directories.)
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Now consider the minimal deadlock. Each process is blocked on
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attempt to acquire some lock and already holds at least one lock. Let's
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consider the set of contended locks. First of all, filesystem lock is
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not contended, since any process blocked on it is not holding any locks.
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Thus all processes are blocked on ->i_rwsem.
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By (3), any process holding a non-directory lock can only be
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waiting on another non-directory lock with a larger address. Therefore
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the process holding the "largest" such lock can always make progress, and
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non-directory objects are not included in the set of contended locks.
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Thus link creation can't be a part of deadlock - it can't be
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blocked on source and it means that it doesn't hold any locks.
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Any contended object is either held by cross-directory rename or
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has a child that is also contended. Indeed, suppose that it is held by
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operation other than cross-directory rename. Then the lock this operation
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is blocked on belongs to child of that object due to (1).
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It means that one of the operations is cross-directory rename.
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Otherwise the set of contended objects would be infinite - each of them
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would have a contended child and we had assumed that no object is its
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own descendent. Moreover, there is exactly one cross-directory rename
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(see above).
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Consider the object blocking the cross-directory rename. One
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of its descendents is locked by cross-directory rename (otherwise we
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would again have an infinite set of contended objects). But that
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means that cross-directory rename is taking locks out of order. Due
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to (2) the order hadn't changed since we had acquired filesystem lock.
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But locking rules for cross-directory rename guarantee that we do not
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try to acquire lock on descendent before the lock on ancestor.
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Contradiction. I.e. deadlock is impossible. Q.E.D.
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These operations are guaranteed to avoid loop creation. Indeed,
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the only operation that could introduce loops is cross-directory rename.
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Since the only new (parent, child) pair added by rename() is (new parent,
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source), such loop would have to contain these objects and the rest of it
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would have to exist before rename(). I.e. at the moment of loop creation
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rename() responsible for that would be holding filesystem lock and new parent
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would have to be equal to or a descendent of source. But that means that
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new parent had been equal to or a descendent of source since the moment when
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we had acquired filesystem lock and rename() would fail with -ELOOP in that
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case.
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While this locking scheme works for arbitrary DAGs, it relies on
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ability to check that directory is a descendent of another object. Current
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implementation assumes that directory graph is a tree. This assumption is
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also preserved by all operations (cross-directory rename on a tree that would
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not introduce a cycle will leave it a tree and link() fails for directories).
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Notice that "directory" in the above == "anything that might have
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children", so if we are going to introduce hybrid objects we will need
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either to make sure that link(2) doesn't work for them or to make changes
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in is_subdir() that would make it work even in presence of such beasts.
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