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qemu: THREADS.txt: rSTize and move to knowledge-base
Move the internal documentation about qemu threading to the knowledge base. The conversion included rstizing of the text document, mainly just fixing of the headline and enclosing function names and code examples into code block sections. Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Ján Tomko <jtomko@redhat.com>
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Peter Krempa
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@ -101,3 +101,6 @@ Internals
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`RPC protocol & APIs <internals/rpc.html>`__
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RPC protocol information and API / dispatch guide
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`QEMU driver threading <internals/qemu-threads.html>`__
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Basics of locking and threaded access to qemu driver primitives.
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@ -5,6 +5,7 @@ docs_kbase_internals_files = [
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'locking',
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'migration',
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'overview',
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'qemu-threads',
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'rpc',
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]
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@ -1,5 +1,7 @@
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QEMU Driver Threading: The Rules
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=================================
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QEMU Driver Threading: The Rules
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================================
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.. contents::
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This document describes how thread safety is ensured throughout
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the QEMU driver. The criteria for this model are:
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@ -8,37 +10,36 @@ the QEMU driver. The criteria for this model are:
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- Code which sleeps must be able to time out after suitable period
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- Must be safe against dispatch of asynchronous events from monitor
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Basic locking primitives
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------------------------
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There are a number of locks on various objects
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* virQEMUDriver *
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``virQEMUDriver``
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The qemu_conf.h file has inline comments describing the locking
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The ``qemu_conf.h`` file has inline comments describing the locking
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needs for each field. Any field marked immutable, self-locking
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can be accessed without the driver lock. For other fields there
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are typically helper APIs in qemu_conf.c that provide serialized
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access to the data. No code outside qemu_conf.c should ever
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are typically helper APIs in ``qemu_conf.c`` that provide serialized
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access to the data. No code outside ``qemu_conf.c`` should ever
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acquire this lock
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* virDomainObj *
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``virDomainObj``
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Will be locked and the reference counter will be increased after calling
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any of the virDomainObjListFindBy{ID,Name,UUID} methods. The preferred way
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any of the ``virDomainObjListFindBy{ID,Name,UUID}`` methods. The preferred way
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of decrementing the reference counter and unlocking the domain is using the
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virDomainObjEndAPI() function.
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``virDomainObjEndAPI()`` function.
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Lock must be held when changing/reading any variable in the virDomainObj *
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Lock must be held when changing/reading any variable in the ``virDomainObj``
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This lock must not be held for anything which sleeps/waits (i.e. monitor
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commands).
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* qemuMonitorPrivatePtr: Job conditions
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``qemuMonitorPrivatePtr`` job conditions
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Since virDomainObj *lock must not be held during sleeps, the job
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Since ``virDomainObj`` lock must not be held during sleeps, the job
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conditions provide additional protection for code making updates.
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QEMU driver uses three kinds of job conditions: asynchronous, agent
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@ -61,30 +62,30 @@ There are a number of locks on various objects
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Agent job condition is then used when thread wishes to talk to qemu
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agent monitor. It is possible to acquire just agent job
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(qemuDomainObjBeginAgentJob), or only normal job (qemuDomainObjBeginJob)
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(``qemuDomainObjBeginAgentJob``), or only normal job (``qemuDomainObjBeginJob``)
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but not both at the same time. Holding an agent job and a normal job would
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allow an unresponsive or malicious agent to block normal libvirt API and
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potentially result in a denial of service. Which type of job to grab
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depends whether caller wishes to communicate only with agent socket, or
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only with qemu monitor socket.
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Immediately after acquiring the virDomainObj *lock, any method
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Immediately after acquiring the ``virDomainObj`` lock, any method
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which intends to update state must acquire asynchronous, normal or
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agent job . The virDomainObj *lock is released while blocking on
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agent job . The ``virDomainObj`` lock is released while blocking on
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these condition variables. Once the job condition is acquired, a
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method can safely release the virDomainObj *lock whenever it hits
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method can safely release the ``virDomainObj`` lock whenever it hits
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a piece of code which may sleep/wait, and re-acquire it after the
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sleep/wait. Whenever an asynchronous job wants to talk to the
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monitor, it needs to acquire nested job (a special kind of normal
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job) to obtain exclusive access to the monitor.
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Since the virDomainObj *lock was dropped while waiting for the
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Since the ``virDomainObj`` lock was dropped while waiting for the
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job condition, it is possible that the domain is no longer active
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when the condition is finally obtained. The monitor lock is only
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safe to grab after verifying that the domain is still active.
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* qemuMonitor *: Mutex
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``qemuMonitor`` mutex
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Lock to be used when invoking any monitor command to ensure safety
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wrt any asynchronous events that may be dispatched from the monitor.
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@ -92,118 +93,111 @@ There are a number of locks on various objects
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The job condition *MUST* be held before acquiring the monitor lock
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The virDomainObj *lock *MUST* be held before acquiring the monitor
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The ``virDomainObj`` lock *MUST* be held before acquiring the monitor
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lock.
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The virDomainObj *lock *MUST* then be released when invoking the
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The ``virDomainObj`` lock *MUST* then be released when invoking the
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monitor command.
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Helper methods
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--------------
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To lock the virDomainObj *
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To lock the ``virDomainObj``
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virObjectLock()
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- Acquires the virDomainObj *lock
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virObjectUnlock()
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- Releases the virDomainObj *lock
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``virObjectLock()``
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- Acquires the ``virDomainObj`` lock
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``virObjectUnlock()``
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- Releases the ``virDomainObj`` lock
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To acquire the normal job condition
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qemuDomainObjBeginJob()
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``qemuDomainObjBeginJob()``
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- Waits until the job is compatible with current async job or no
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async job is running
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- Waits for job.cond condition 'job.active != 0' using virDomainObj *
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- Waits for ``job.cond`` condition ``job.active != 0`` using ``virDomainObj``
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mutex
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- Rechecks if the job is still compatible and repeats waiting if it
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isn't
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- Sets job.active to the job type
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- Sets ``job.active`` to the job type
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qemuDomainObjEndJob()
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``qemuDomainObjEndJob()``
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- Sets job.active to 0
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- Signals on job.cond condition
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To acquire the agent job condition
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qemuDomainObjBeginAgentJob()
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``qemuDomainObjBeginAgentJob()``
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- Waits until there is no other agent job set
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- Sets job.agentActive tp the job type
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qemuDomainObjEndAgentJob()
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- Sets job.agentActive to 0
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- Signals on job.cond condition
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- Sets ``job.agentActive`` to the job type
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``qemuDomainObjEndAgentJob()``
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- Sets ``job.agentActive`` to 0
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- Signals on ``job.cond`` condition
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To acquire the asynchronous job condition
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qemuDomainObjBeginAsyncJob()
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``qemuDomainObjBeginAsyncJob()``
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- Waits until no async job is running
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- Waits for job.cond condition 'job.active != 0' using virDomainObj *
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- Waits for ``job.cond`` condition ``job.active != 0`` using ``virDomainObj``
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mutex
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- Rechecks if any async job was started while waiting on job.cond
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- Rechecks if any async job was started while waiting on ``job.cond``
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and repeats waiting in that case
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- Sets job.asyncJob to the asynchronous job type
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qemuDomainObjEndAsyncJob()
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- Sets job.asyncJob to 0
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- Broadcasts on job.asyncCond condition
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- Sets ``job.asyncJob`` to the asynchronous job type
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``qemuDomainObjEndAsyncJob()``
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- Sets ``job.asyncJob`` to 0
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- Broadcasts on ``job.asyncCond`` condition
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To acquire the QEMU monitor lock
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qemuDomainObjEnterMonitor()
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- Acquires the qemuMonitorObjPtr lock
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- Releases the virDomainObj *lock
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``qemuDomainObjEnterMonitor()``
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- Acquires the ``qemuMonitorObj`` lock
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- Releases the ``virDomainObj`` lock
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qemuDomainObjExitMonitor()
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- Releases the qemuMonitorObjPtr lock
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- Acquires the virDomainObj *lock
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``qemuDomainObjExitMonitor()``
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- Releases the ``qemuMonitorObj`` lock
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- Acquires the ``virDomainObj`` lock
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These functions must not be used by an asynchronous job.
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To acquire the QEMU monitor lock as part of an asynchronous job
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qemuDomainObjEnterMonitorAsync()
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``qemuDomainObjEnterMonitorAsync()``
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- Validates that the right async job is still running
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- Acquires the qemuMonitorObjPtr lock
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- Releases the virDomainObj *lock
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- Acquires the ``qemuMonitorObj`` lock
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- Releases the ``virDomainObj`` lock
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- Validates that the VM is still active
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qemuDomainObjExitMonitor()
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- Releases the qemuMonitorObjPtr lock
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- Acquires the virDomainObj *lock
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- Releases the ``qemuMonitorObj`` lock
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- Acquires the ``virDomainObj`` lock
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These functions are for use inside an asynchronous job; the caller
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must check for a return of -1 (VM not running, so nothing to exit).
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Helper functions may also call this with VIR_ASYNC_JOB_NONE when
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Helper functions may also call this with ``VIR_ASYNC_JOB_NONE`` when
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used from a sync job (such as when first starting a domain).
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To keep a domain alive while waiting on a remote command
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qemuDomainObjEnterRemote()
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- Releases the virDomainObj *lock
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``qemuDomainObjEnterRemote()``
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- Releases the ``virDomainObj`` lock
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qemuDomainObjExitRemote()
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- Acquires the virDomainObj *lock
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``qemuDomainObjExitRemote()``
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- Acquires the ``virDomainObj`` lock
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Design patterns
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---------------
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* Accessing something directly to do with a virDomainObj *
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* Accessing something directly to do with a ``virDomainObj``::
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virDomainObj *obj;
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@ -214,7 +208,7 @@ Design patterns
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virDomainObjEndAPI(&obj);
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* Updating something directly to do with a virDomainObj *
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* Updating something directly to do with a ``virDomainObj``::
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virDomainObj *obj;
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@ -229,7 +223,7 @@ Design patterns
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virDomainObjEndAPI(&obj);
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* Invoking a monitor command on a virDomainObj *
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* Invoking a monitor command on a ``virDomainObj``::
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virDomainObj *obj;
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qemuDomainObjPrivate *priv;
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@ -252,7 +246,7 @@ Design patterns
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virDomainObjEndAPI(&obj);
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* Invoking an agent command on a virDomainObj *
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* Invoking an agent command on a ``virDomainObj``::
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virDomainObj *obj;
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qemuAgent *agent;
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@ -276,7 +270,7 @@ Design patterns
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virDomainObjEndAPI(&obj);
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* Running asynchronous job
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* Running asynchronous job::
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virDomainObj *obj;
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qemuDomainObjPrivate *priv;
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@ -316,7 +310,7 @@ Design patterns
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virDomainObjEndAPI(&obj);
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* Coordinating with a remote server for migration
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* Coordinating with a remote server for migration::
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virDomainObj *obj;
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qemuDomainObjPrivate *priv;
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