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