Commit 0f933d15 authored by Guido Trotter's avatar Guido Trotter

Add doc/locking.txt, documenting locking order

Reviewed-by: imsnah
parent 082c5adb
......@@ -139,6 +139,7 @@ EXTRA_DIST = \
doc/examples/ganeti.initd.in \
doc/examples/ganeti.cron.in \
doc/examples/dumb-allocator \
doc/locking.txt \
test/testutils.py \
test/mocks.py \
$(dist_TESTS) \
......
Introduction:
This document describes lock order dependencies in Ganeti.
It is divided by functional sections
Opcode Execution Locking:
These locks are declared by Logical Units (LUs) (in cmdlib.py) and acquired by
the Processor (in mcpu.py) with the aid of the Ganeti Locking Library
(locking.py). They are acquired in the following order:
* BGL: this is the Big Ganeti Lock, it exists for retrocompatibility. New LUs
acquire it in a shared fashion, and are able to execute all toghether
(baring other lock waits) while old LUs acquire it exclusively and can only
execute one at a time, and not at the same time with new LUs.
* Instance locks: can be declared in ExpandNames() o DeclareLocks() by an LU,
and have the same name as the instance itself. They are acquired as a set.
Internally the locking library acquired them in alphabetical order.
* Node locks: can be declared in ExpandNames() o DeclareLocks() by an LU, and
have the same name as the node itself. They are acquired as a set.
Internally the locking library acquired them in alphabetical order. Given
this order it's possible to safely acquire a set of instances, and then the
nodes they reside on.
The ConfigWriter (in config.py) is also protected by a SharedLock, which is
shared by functions that read the config and acquired exclusively by functions
that modify it. Since the ConfigWriter calls rpc.call_upload_file to all nodes
to distribute the config without holding the node locks, this call must be able
to execute on the nodes in parallel with other operations (but not necessarily
concurrently with itself on the same file, as inside the ConfigWriter this is
called with the internal config lock held.
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