[PATCH 1/3] qemu: THREADS.txt: rSTize and move to knowledge-base
Peter Krempa
pkrempa at redhat.com
Mon May 16 15:51:24 UTC 2022
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 at redhat.com>
---
docs/kbase/index.rst | 3 +
docs/kbase/internals/meson.build | 1 +
.../kbase/internals/qemu-threads.rst | 136 +++++++++---------
3 files changed, 69 insertions(+), 71 deletions(-)
rename src/qemu/THREADS.txt => docs/kbase/internals/qemu-threads.rst (70%)
diff --git a/docs/kbase/index.rst b/docs/kbase/index.rst
index 0848467d51..f1cd143fab 100644
--- a/docs/kbase/index.rst
+++ b/docs/kbase/index.rst
@@ -101,3 +101,6 @@ Internals
`RPC protocol & APIs <internals/rpc.html>`__
RPC protocol information and API / dispatch guide
+
+`QEMU driver threading <internals/qemu-threads.html>`__
+ Basics of locking and threaded access to qemu driver primitives.
diff --git a/docs/kbase/internals/meson.build b/docs/kbase/internals/meson.build
index 8b5daad1f9..3e84b398b2 100644
--- a/docs/kbase/internals/meson.build
+++ b/docs/kbase/internals/meson.build
@@ -5,6 +5,7 @@ docs_kbase_internals_files = [
'locking',
'migration',
'overview',
+ 'qemu-threads',
'rpc',
]
diff --git a/src/qemu/THREADS.txt b/docs/kbase/internals/qemu-threads.rst
similarity index 70%
rename from src/qemu/THREADS.txt
rename to docs/kbase/internals/qemu-threads.rst
index b5f54f203c..c68512d1b3 100644
--- a/src/qemu/THREADS.txt
+++ b/docs/kbase/internals/qemu-threads.rst
@@ -1,5 +1,7 @@
- QEMU Driver Threading: The Rules
- =================================
+QEMU Driver Threading: The Rules
+================================
+
+.. contents::
This document describes how thread safety is ensured throughout
the QEMU driver. The criteria for this model are:
@@ -8,37 +10,36 @@ the QEMU driver. The criteria for this model are:
- Code which sleeps must be able to time out after suitable period
- Must be safe against dispatch of asynchronous events from monitor
-
Basic locking primitives
------------------------
There are a number of locks on various objects
- * virQEMUDriver *
+ ``virQEMUDriver``
- The qemu_conf.h file has inline comments describing the locking
+ The ``qemu_conf.h`` file has inline comments describing the locking
needs for each field. Any field marked immutable, self-locking
can be accessed without the driver lock. For other fields there
- are typically helper APIs in qemu_conf.c that provide serialized
- access to the data. No code outside qemu_conf.c should ever
+ are typically helper APIs in ``qemu_conf.c`` that provide serialized
+ access to the data. No code outside ``qemu_conf.c`` should ever
acquire this lock
- * virDomainObj *
+ ``virDomainObj``
Will be locked and the reference counter will be increased after calling
- any of the virDomainObjListFindBy{ID,Name,UUID} methods. The preferred way
+ any of the ``virDomainObjListFindBy{ID,Name,UUID}`` methods. The preferred way
of decrementing the reference counter and unlocking the domain is using the
- virDomainObjEndAPI() function.
+ ``virDomainObjEndAPI()`` function.
- Lock must be held when changing/reading any variable in the virDomainObj *
+ Lock must be held when changing/reading any variable in the ``virDomainObj``
This lock must not be held for anything which sleeps/waits (i.e. monitor
commands).
- * qemuMonitorPrivatePtr: Job conditions
+ ``qemuMonitorPrivatePtr`` job conditions
- Since virDomainObj *lock must not be held during sleeps, the job
+ Since ``virDomainObj`` lock must not be held during sleeps, the job
conditions provide additional protection for code making updates.
QEMU driver uses three kinds of job conditions: asynchronous, agent
@@ -61,30 +62,30 @@ There are a number of locks on various objects
Agent job condition is then used when thread wishes to talk to qemu
agent monitor. It is possible to acquire just agent job
- (qemuDomainObjBeginAgentJob), or only normal job (qemuDomainObjBeginJob)
+ (``qemuDomainObjBeginAgentJob``), or only normal job (``qemuDomainObjBeginJob``)
but not both at the same time. Holding an agent job and a normal job would
allow an unresponsive or malicious agent to block normal libvirt API and
potentially result in a denial of service. Which type of job to grab
depends whether caller wishes to communicate only with agent socket, or
only with qemu monitor socket.
- Immediately after acquiring the virDomainObj *lock, any method
+ Immediately after acquiring the ``virDomainObj`` lock, any method
which intends to update state must acquire asynchronous, normal or
- agent job . The virDomainObj *lock is released while blocking on
+ agent job . The ``virDomainObj`` lock is released while blocking on
these condition variables. Once the job condition is acquired, a
- method can safely release the virDomainObj *lock whenever it hits
+ method can safely release the ``virDomainObj`` lock whenever it hits
a piece of code which may sleep/wait, and re-acquire it after the
sleep/wait. Whenever an asynchronous job wants to talk to the
monitor, it needs to acquire nested job (a special kind of normal
job) to obtain exclusive access to the monitor.
- Since the virDomainObj *lock was dropped while waiting for the
+ Since the ``virDomainObj`` lock was dropped while waiting for the
job condition, it is possible that the domain is no longer active
when the condition is finally obtained. The monitor lock is only
safe to grab after verifying that the domain is still active.
- * qemuMonitor *: Mutex
+ ``qemuMonitor`` mutex
Lock to be used when invoking any monitor command to ensure safety
wrt any asynchronous events that may be dispatched from the monitor.
@@ -92,118 +93,111 @@ There are a number of locks on various objects
The job condition *MUST* be held before acquiring the monitor lock
- The virDomainObj *lock *MUST* be held before acquiring the monitor
+ The ``virDomainObj`` lock *MUST* be held before acquiring the monitor
lock.
- The virDomainObj *lock *MUST* then be released when invoking the
+ The ``virDomainObj`` lock *MUST* then be released when invoking the
monitor command.
Helper methods
--------------
-To lock the virDomainObj *
-
- virObjectLock()
- - Acquires the virDomainObj *lock
+To lock the ``virDomainObj``
- virObjectUnlock()
- - Releases the virDomainObj *lock
+ ``virObjectLock()``
+ - Acquires the ``virDomainObj`` lock
+ ``virObjectUnlock()``
+ - Releases the ``virDomainObj`` lock
To acquire the normal job condition
- qemuDomainObjBeginJob()
+ ``qemuDomainObjBeginJob()``
- Waits until the job is compatible with current async job or no
async job is running
- - Waits for job.cond condition 'job.active != 0' using virDomainObj *
+ - Waits for ``job.cond`` condition ``job.active != 0`` using ``virDomainObj``
mutex
- Rechecks if the job is still compatible and repeats waiting if it
isn't
- - Sets job.active to the job type
+ - Sets ``job.active`` to the job type
-
- qemuDomainObjEndJob()
+ ``qemuDomainObjEndJob()``
- Sets job.active to 0
- Signals on job.cond condition
-
To acquire the agent job condition
- qemuDomainObjBeginAgentJob()
+ ``qemuDomainObjBeginAgentJob()``
- Waits until there is no other agent job set
- - Sets job.agentActive tp the job type
-
- qemuDomainObjEndAgentJob()
- - Sets job.agentActive to 0
- - Signals on job.cond condition
+ - Sets ``job.agentActive`` to the job type
+ ``qemuDomainObjEndAgentJob()``
+ - Sets ``job.agentActive`` to 0
+ - Signals on ``job.cond`` condition
To acquire the asynchronous job condition
- qemuDomainObjBeginAsyncJob()
+ ``qemuDomainObjBeginAsyncJob()``
- Waits until no async job is running
- - Waits for job.cond condition 'job.active != 0' using virDomainObj *
+ - Waits for ``job.cond`` condition ``job.active != 0`` using ``virDomainObj``
mutex
- - Rechecks if any async job was started while waiting on job.cond
+ - Rechecks if any async job was started while waiting on ``job.cond``
and repeats waiting in that case
- - Sets job.asyncJob to the asynchronous job type
-
-
- qemuDomainObjEndAsyncJob()
- - Sets job.asyncJob to 0
- - Broadcasts on job.asyncCond condition
+ - Sets ``job.asyncJob`` to the asynchronous job type
+ ``qemuDomainObjEndAsyncJob()``
+ - Sets ``job.asyncJob`` to 0
+ - Broadcasts on ``job.asyncCond`` condition
To acquire the QEMU monitor lock
- qemuDomainObjEnterMonitor()
- - Acquires the qemuMonitorObjPtr lock
- - Releases the virDomainObj *lock
+ ``qemuDomainObjEnterMonitor()``
+ - Acquires the ``qemuMonitorObj`` lock
+ - Releases the ``virDomainObj`` lock
- qemuDomainObjExitMonitor()
- - Releases the qemuMonitorObjPtr lock
- - Acquires the virDomainObj *lock
+ ``qemuDomainObjExitMonitor()``
+ - Releases the ``qemuMonitorObj`` lock
+ - Acquires the ``virDomainObj`` lock
These functions must not be used by an asynchronous job.
To acquire the QEMU monitor lock as part of an asynchronous job
- qemuDomainObjEnterMonitorAsync()
+ ``qemuDomainObjEnterMonitorAsync()``
- Validates that the right async job is still running
- - Acquires the qemuMonitorObjPtr lock
- - Releases the virDomainObj *lock
+ - Acquires the ``qemuMonitorObj`` lock
+ - Releases the ``virDomainObj`` lock
- Validates that the VM is still active
qemuDomainObjExitMonitor()
- - Releases the qemuMonitorObjPtr lock
- - Acquires the virDomainObj *lock
+ - Releases the ``qemuMonitorObj`` lock
+ - Acquires the ``virDomainObj`` lock
These functions are for use inside an asynchronous job; the caller
must check for a return of -1 (VM not running, so nothing to exit).
- Helper functions may also call this with VIR_ASYNC_JOB_NONE when
+ Helper functions may also call this with ``VIR_ASYNC_JOB_NONE`` when
used from a sync job (such as when first starting a domain).
To keep a domain alive while waiting on a remote command
- qemuDomainObjEnterRemote()
- - Releases the virDomainObj *lock
+ ``qemuDomainObjEnterRemote()``
+ - Releases the ``virDomainObj`` lock
- qemuDomainObjExitRemote()
- - Acquires the virDomainObj *lock
+ ``qemuDomainObjExitRemote()``
+ - Acquires the ``virDomainObj`` lock
Design patterns
---------------
-
- * Accessing something directly to do with a virDomainObj *
+ * Accessing something directly to do with a ``virDomainObj``::
virDomainObj *obj;
@@ -214,7 +208,7 @@ Design patterns
virDomainObjEndAPI(&obj);
- * Updating something directly to do with a virDomainObj *
+ * Updating something directly to do with a ``virDomainObj``::
virDomainObj *obj;
@@ -229,7 +223,7 @@ Design patterns
virDomainObjEndAPI(&obj);
- * Invoking a monitor command on a virDomainObj *
+ * Invoking a monitor command on a ``virDomainObj``::
virDomainObj *obj;
qemuDomainObjPrivate *priv;
@@ -252,7 +246,7 @@ Design patterns
virDomainObjEndAPI(&obj);
- * Invoking an agent command on a virDomainObj *
+ * Invoking an agent command on a ``virDomainObj``::
virDomainObj *obj;
qemuAgent *agent;
@@ -276,7 +270,7 @@ Design patterns
virDomainObjEndAPI(&obj);
- * Running asynchronous job
+ * Running asynchronous job::
virDomainObj *obj;
qemuDomainObjPrivate *priv;
@@ -316,7 +310,7 @@ Design patterns
virDomainObjEndAPI(&obj);
- * Coordinating with a remote server for migration
+ * Coordinating with a remote server for migration::
virDomainObj *obj;
qemuDomainObjPrivate *priv;
--
2.35.3
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