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Ganeti locking

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Introduction

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This document describes lock order dependencies in Ganeti.

It is divided by functional sections

Opcode Execution Locking

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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.

Job Queue Locking

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

functions can be called from any thread. One must only pay attention not to

call the job queue from functions called by the queue itself (e.g. logical

units).

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

During development we tried to split locks, but demmed it to be too dangerous

and difficicult at the time. Once the lock is held, all functions called must

not grab the queue lock again (e.g. call only functions with a ``Unlocked``

suffix).

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

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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.