Docs: Document reference counting scheme implemented by Object

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>

diff --git a/configure.ac b/configure.ac
index f23e460b757b9e5514bac40a1860a61b695afc78..a6be47834524e684bb61818e039cf11d60d5da63 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -304,6 +304,7 @@ AC_CONFIG_FILES([
        doc/Makefile
        doc/bindings/Makefile
        doc/bindings/python/Makefile
+       doc/images/Makefile
        lib/Makefile
        lib/prio_heap/Makefile
        include/Makefile

diff --git a/doc/Makefile.am b/doc/Makefile.am
index f32a4dd263b0126135651a6fba1a6d94989491d8..0ef5e20e8401edabfe044a8e303a0f4807ea468c 100644 (file)
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -1,7 +1,7 @@
-SUBDIRS = bindings
+SUBDIRS = bindings images
 
 dist_man_MANS = babeltrace.1 babeltrace-log.1
 
-dist_doc_DATA = API.txt lttng-live.txt
+dist_doc_DATA = API.txt lttng-live.txt ref-counting.md
 
 EXTRA_DIST = development.txt

diff --git a/doc/images/Makefile.am b/doc/images/Makefile.am
new file mode 100644 (file)
index 0000000..05d138b
--- /dev/null
+++ b/doc/images/Makefile.am
@@ -0,0 +1,4 @@
+dist_doc_DATA = bt-ref01.svg bt-ref02.svg bt-ref03.svg bt-ref04.svg \
+       bt-ref05.svg bt-ref06.svg bt-ref07.svg bt-ref08.svg \
+       bt-ref09.svg bt-ref10.svg bt-ref11.svg bt-ref12.svg \
+       bt-ref13.svg

diff --git a/doc/images/bt-ref01.svg b/doc/images/bt-ref01.svg
new file mode 100644 (file)
index 0000000..7291f2c
--- /dev/null
+++ b/doc/images/bt-ref01.svg
@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="433px" height="278px" version="1.1" style="background-color: rgb(255, 255, 255);"><defs/><g transform="translate(0.5,0.5)"><ellipse cx="196" cy="46" rx="30" ry="30" fill="#ffd966" stroke="#d6b656" pointer-events="none"/><g transform="translate(178,40)"><switch><foreignObject pointer-events="all" width="35" height="16" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; font-size: 12px; font-family: Helvetica; color: rgb(0, 0, 0); line-height: 1.2; vertical-align: top; width: 35px; white-space: normal; text-align: center;"><div xmlns="http://www.w3.org/1999/xhtml" style="display:inline-block;text-align:inherit;text-decoration:inherit;">TC1</div></div></foreignObject><text x="18" y="14" fill="#000000" text-anchor="middle" font-size="12px" font-family="Helvetica">[Not supported by viewer]</text></switch></g><ellipse cx="146" cy="146" rx="30" ry="30" fill="#ea6b66" stroke="#b85450" pointer-events="none"/><g transform="translate(128,140)"><switch><foreignObject pointer-events="all" width="36" height="16" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; 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\ No newline at end of file

diff --git a/doc/images/bt-ref02.svg b/doc/images/bt-ref02.svg
new file mode 100644 (file)
index 0000000..da1eb73
--- /dev/null
+++ b/doc/images/bt-ref02.svg
@@ -0,0 +1,3 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="433px" height="278px" version="1.1" style="background-color: rgb(255, 255, 255);"><defs/><g transform="translate(0.5,0.5)"><ellipse cx="196" cy="46" rx="30" ry="30" fill="#ffd966" stroke="#d6b656" pointer-events="none"/><g transform="translate(178,40)"><switch><foreignObject pointer-events="all" width="35" height="16" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; font-size: 12px; font-family: Helvetica; color: rgb(0, 0, 0); line-height: 1.2; vertical-align: top; width: 35px; white-space: normal; text-align: center;"><div xmlns="http://www.w3.org/1999/xhtml" style="display:inline-block;text-align:inherit;text-decoration:inherit;">TC1</div></div></foreignObject><text x="18" y="14" fill="#000000" text-anchor="middle" font-size="12px" font-family="Helvetica">[Not supported by viewer]</text></switch></g><ellipse cx="146" cy="146" rx="30" ry="30" fill="#ea6b66" stroke="#b85450" pointer-events="none"/><g transform="translate(128,140)"><switch><foreignObject pointer-events="all" width="36" height="16" requiredFeatures="http://www.w3.org/TR/SVG11/feature#Extensibility"><div xmlns="http://www.w3.org/1999/xhtml" style="display: inline-block; 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diff --git a/doc/images/bt-ref03.svg b/doc/images/bt-ref03.svg
new file mode 100644 (file)
index 0000000..12f8a14
--- /dev/null
+++ b/doc/images/bt-ref03.svg
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diff --git a/doc/images/bt-ref04.svg b/doc/images/bt-ref04.svg
new file mode 100644 (file)
index 0000000..10ada1d
--- /dev/null
+++ b/doc/images/bt-ref04.svg
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diff --git a/doc/images/bt-ref05.svg b/doc/images/bt-ref05.svg
new file mode 100644 (file)
index 0000000..d14a01c
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+++ b/doc/images/bt-ref05.svg
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diff --git a/doc/images/bt-ref06.svg b/doc/images/bt-ref06.svg
new file mode 100644 (file)
index 0000000..4f8328f
--- /dev/null
+++ b/doc/images/bt-ref06.svg
@@ -0,0 +1,3 @@
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diff --git a/doc/images/bt-ref07.svg b/doc/images/bt-ref07.svg
new file mode 100644 (file)
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diff --git a/doc/images/bt-ref08.svg b/doc/images/bt-ref08.svg
new file mode 100644 (file)
index 0000000..e813cee
--- /dev/null
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diff --git a/doc/images/bt-ref09.svg b/doc/images/bt-ref09.svg
new file mode 100644 (file)
index 0000000..98a7ad4
--- /dev/null
+++ b/doc/images/bt-ref09.svg
@@ -0,0 +1,3 @@
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diff --git a/doc/images/bt-ref10.svg b/doc/images/bt-ref10.svg
new file mode 100644 (file)
index 0000000..5bad355
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+++ b/doc/images/bt-ref10.svg
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+<?xml version="1.0" encoding="UTF-8"?>
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diff --git a/doc/images/bt-ref11.svg b/doc/images/bt-ref11.svg
new file mode 100644 (file)
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diff --git a/doc/images/bt-ref12.svg b/doc/images/bt-ref12.svg
new file mode 100644 (file)
index 0000000..b6f6b89
--- /dev/null
+++ b/doc/images/bt-ref12.svg
@@ -0,0 +1,3 @@
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diff --git a/doc/images/bt-ref13.svg b/doc/images/bt-ref13.svg
new file mode 100644 (file)
index 0000000..de88b29
--- /dev/null
+++ b/doc/images/bt-ref13.svg
@@ -0,0 +1,3 @@
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diff --git a/doc/ref-counting.md b/doc/ref-counting.md
new file mode 100644 (file)
index 0000000..32b8608
--- /dev/null
+++ b/doc/ref-counting.md
@@ -0,0 +1,195 @@
+# Babeltrace Object Reference Counting and Lifetime
+
+This document covers the rationale behind the design of Babeltrace's object
+lifetime management.
+
+Starting from Babeltrace 2.x, all publicly exposed objects inherit a common
+base: bt_object. This base provides a number of facilities to all objects, chief
+amongst which are lifetime management functions.
+
+The reference count of all public objects is controlled by invoking
+the `bt_get()` and `bt_put()` functions which respectively
+increment and decrement an object's reference count.
+
+As far as lifetime management in concerned, Babeltrace makes a clear
+distinction between regular objects, which have a single parent, and root
+objects, which don't.
+
+## The Problem
+
+Let us consider a problematic case to illustrate the need for this
+distinction.
+
+A user of the CTF Writer library declares a Trace, which *has* a
+Stream Class (the declaration of a stream) and that Stream Class, in
+turn, *has* an Event Class (the declaration of an event).
+
+Nothing prevents this user from releasing his reference on any one of
+these objects in any order. However, all objects in the "Trace <->
+Stream <-> Event" hierarchy can be retrieved from any other.
+
+For instance, the user could discard his reference on both the Event
+Class and the Stream Class, only keeping a reference on the Trace.
+From this Trace reference, Stream Classes can be enumerated, providing
+the user with a new reference to the Stream Class he discarded
+earlier. Event Classes can also be enumerated from Stream Classes,
+providing the user with references to the individual Event Classes.
+
+Conversely, the user could also hold a reference to an Event Class and
+retrieve its parent Stream Class. The Trace, in turn, can then be
+retrieved from the Stream Class.
+
+This example illustrates what could be interpreted as a circular
+reference dependency existing between these objects. Of course, if the
+objects in such a scenario were to hold references to each other (in
+both directions), we would be in presence of a circular ownership
+resulting in a leak of both objects as their reference counts would
+never reach zero.
+
+Nonetheless, the API must offer the guarantee that holding a node to any node of
+the graph keeps all other reachable nodes alive.
+
+## The Solution
+
+The scheme employed in Babeltrace to break this cycle consists in the
+"children" holding *Reverse Component References* to their parents.  That
+is, in the context of CTF-IR, that Event Classes hold a reference to
+their Stream Class and Stream Classes hold a reference to their Trace.
+
+On the other hand, parents hold *Claiming Aggregation References* to
+their children. A claiming aggregation reference means that the
+object being referenced should not be deleted as long as the reference
+still exists. In this respect, it can be said that parents truly hold the
+ownership of their children, since they control their lifetime. Conversely,
+the reference counting mechanism is leveraged by children to notify parents
+that no other child indirectly exposes the parent.
+
+When a parented object's reference count reaches zero, it invokes
+`bt_put()` on its parent and does **not** free itself. However, from that
+point, the object depends on its parent to signal the moment when it
+can be safely reclaimed.
+
+The invocation of `bt_put()` by the last children holding a reference to its
+parent might trigger a cascade of `bt_put()` from child to parent. Eventually,
+a **root** object is reached. At that point, if this orphaned object's
+reference count reaches zero, the object will invoke the `destroy()` method
+defined by every one of its children as part of their base `struct bt_object`.
+The key point here is that the cascade of `destroy()` will necessarily originate
+from the root and propagate in pre-order to the children. These children will
+propagate the destruction to their own children before reclaiming their own
+memory. This ensures that a node's pointer to its parent is *always* valid
+since the parent has the responsibility of tearing-down their children before
+cleaning themselves-up.
+
+Assuming a reference to an object is *acquired* by calling `bt_get()` while its
+reference count is zero, the object will, in turn, acquire a reference on its
+parent using `bt_get()`. At that point, the child can be thought of as having
+converted its weak reference to its parent into a regular reference. That is
+why this reference is referred to as a *claiming* aggregation reference.
+
+## Caveats
+
+This scheme imposes a number of strict rules defining the relation
+between objects:
+
+  * Objects may only have one parent,
+  * Objects, beside the root, are only retrievable from their direct parent or
+    children.
+
+## Walking through an example
+
+The initial situation is rather simple. **User A** is holding a reference to a
+trace, **TC1**. As per the rules previously enounced, Stream Classes **SC1** and
+**SC2** don't hold a reference to **TC1** since their own reference counts are
+zero. The same holds true for **EC1**, **EC2** and **EC3** with respect to
+**SC1** and **SC2**.
+
+![](images/bt-ref01.svg)
+
+In this second step, we can see that User A has acquired a reference on **SC2**
+through the Trace, **TC1**.
+
+The Stream Class' reference count transitions from zero to one, triggering the
+acquisition of a strong reference on **TC1** from **SC2**.
+
+Hence, at this point, the Trace's ownership is shared by **User A** and
+**SC2**.
+
+![](images/bt-ref02.svg)
+
+Next, **User A** acquires a reference on the **EC3** Event Class through its
+parent Stream Class, **SC2**. Again, the transition of an object's reference
+count from 0 to 1 triggers the acquisition of a reference on its parent.
+
+Note that SC2's reference count was incremented to 2. The Trace's reference
+count remains unchanged.
+
+![](images/bt-ref03.svg)
+
+**User A** decides to drop its reference on **SC2**. **SC2**'s reference count
+returns back to 1, everything else remaining unchanged.
+
+![](images/bt-ref04.svg)
+
+**User A** can then decide to drop its reference on the Trace. This results in
+a reversal of the initial situation: **User A** now owns an event, **EC3**,
+which is keeping everything else alive and reachable.
+
+![](images/bt-ref05.svg)
+
+If another object, **User B**, enters the picture and acquires a reference on
+the **SC1** Stream Class, we see that **SC1**'s reference count
+transitioned from 0 to 1, triggering the acquisition of a reference on **TC1**.
+
+![](images/bt-ref06.svg)
+
+**User B** hands off a reference to **EC1**, acquired through **SC1**, to
+another object, **User C**. The acquisition of a reference on **EC1**, which
+transitions from 0 to 1, triggers the acquisition of a reference on its parent,
+**SC1**.
+
+![](images/bt-ref07.svg)
+
+At some point, **User A** releases its reference on **EC3**. Since **EC3**'s
+reference count transitions to zero, it releases its reference on **SC2**.
+**SC2**'s reference count, in turn, reaches zero and it releases its reference
+to **TC1**.
+
+**TC1**'s reference count is now 1 and no further action is taken.
+
+![](images/bt-ref08.svg)
+
+**User B** releases its reference on **SC1**. **User C** becomes the sole owner
+of the whole hierarchy through his ownership of **EC1**.
+
+![](images/bt-ref09.svg)
+
+Finally, **User C** releases his ownership of **EC1**, triggering the release of
+the whole hierarchy. We will walk through the reclamation of the whole graph.
+
+Mirroring what happened when **User A** released its last reference on **EC3**,
+the release of **EC1** by **User C** causes its reference count to fall to zero.
+
+This transition to zero causes **EC1** to release its reference on **SC1**.
+**SC1**'s reference count reaching zero causes it to release its reference on
+**TC1**.
+
+![](images/bt-ref10.svg)
+
+Since the reference count of **TC1**, a root object, has reached zero, it
+invokes the `destroy()` method on its children. This method is recursive and
+causes the Stream Classes to call the `destroy()` method on their Event Classes.
+
+The Event Classes are reached and, having no children of their own, are
+reclaimed.
+
+![](images/bt-ref11.svg)
+
+The Stream Classes having destroyed their children, are then reclaimed by the
+Trace.
+
+![](images/bt-ref12.svg)
+
+Finally, the Stream Classes having been reclaimed, **TC1** is reclaimed.
+
+![](images/bt-ref13.svg)