mgr/volumes: lock module to serialize volume operations

Signed-off-by: Venky Shankar <vshankar@redhat.com>
(cherry picked from commit bc89d05358404039938d260da1a7f0f6a4953e00)

diff --git a/src/pybind/mgr/volumes/fs/operations/__init__.py b/src/pybind/mgr/volumes/fs/operations/__init__.py
new file mode 100644 (file)
index 0000000..e69de29

diff --git a/src/pybind/mgr/volumes/fs/operations/lock.py b/src/pybind/mgr/volumes/fs/operations/lock.py
new file mode 100644 (file)
index 0000000..f9c44dc
--- /dev/null
+++ b/src/pybind/mgr/volumes/fs/operations/lock.py
@@ -0,0 +1,36 @@
+from contextlib import contextmanager
+from threading import Lock
+
+# singleton design pattern taken from http://www.aleax.it/5ep.html
+
+class GlobalLock(object):
+    """
+    Global lock to serialize operations in mgr/volumes. This lock
+    is currently held when accessing (opening) a volume to perform
+    group/subvolume operations. Since this is a big lock, it's rather
+    inefficient -- but right now it's ok since mgr/volumes does not
+    expect concurrent operations via its APIs.
+
+    As and when features get added (such as clone, where mgr/volumes
+    would maintain subvolume states in the filesystem), there might
+    be a need to allow concurrent operations. In that case it would
+    be nice to implement an efficient path based locking mechanism.
+
+    See: https://people.eecs.berkeley.edu/~kubitron/courses/cs262a-F14/projects/reports/project6_report.pdf
+    """
+    _shared_state = {
+        'lock' : Lock(),
+        'init' : False
+    }
+
+    def __init__(self):
+        with self._shared_state['lock']:
+            if not self._shared_state['init']:
+                self._shared_state['init'] = True
+        # share this state among all instances
+        self.__dict__ = self._shared_state
+
+    @contextmanager
+    def lock_op(self):
+        with self._shared_state['lock']:
+            yield