Synnefo » snf-ganeti

Ganeti locking

==============

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() or 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() or 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.

Job Queue Locking

-----------------

The job queue is designed to be thread-safe. This means that its public

functions can be called from any thread. The job queue can be called

from functions called by the queue itself (e.g. logical units), but

special attention must be paid not to create deadlocks or an invalid

state.

The single queue lock is used from all classes involved in the queue

handling.  During development we tried to split locks, but deemed it to

be too dangerous and difficult at the time. Job queue functions

acquiring the lock can be safely called from all the rest of the code,

as the lock is released before leaving the job queue again. Unlocked

functions should only be called from job queue related classes (e.g. in

jqueue.py) and the lock must be acquired beforehand.

In the job queue worker (``_JobQueueWorker``), the lock must be released

before calling the LU processor. Otherwise a deadlock can occur when log

messages are added to opcode results.

Node Daemon Locking

-------------------

The node daemon contains a lock for the job queue. In order to avoid

conflicts and/or corruption when an eventual master daemon or another

node daemon is running, it must be held for all job queue operations

There's one special case for the node daemon running on the master node.

If grabbing the lock in exclusive fails on startup, the code assumes all

checks have been done by the process keeping the lock.

.. vim: set textwidth=72 :