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# 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.

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