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