(*<*)+ −
theory Paper+ −
imports CpsG ExtGG LaTeXsugar+ −
begin+ −
(*>*)+ −
+ −
section {* Introduction *}+ −
+ −
text {*+ −
Many realtime systems need to support processes with priorities and+ −
locking of resources. Locking of resources ensures... Priorities+ −
are needed so that some processes can finish their work within ``hard''+ −
deadlines. Unfortunately both features interact in subtle ways+ −
leading to the problem, called Priority Inversion. Suppose three+ −
processes with priorities $H$(igh), $M$(edium) and $L$(ow). We would+ −
assume that process $H$ cannot be blocked by any process with lower+ −
priority. Unfortunately in a naive implementation, this can happen+ −
and $H$ even can be delayed indefinitely by processes with lower+ −
priorities. For this let $L$ be in the posession of lock for a+ −
research which also $H$ needs. $H$ must therefore wait for $L$ to+ −
release this lock. Unfortunately, $L$ can in turn be blocked by any+ −
process with priority $M$, and so $H$ sits there potentially waiting+ −
indefinitely.+ −
+ −
If this problem of inversion of priorities is left untreated,+ −
systems can become unpredictable and have dire consequences. The+ −
classic example where this happened in practice is the software on+ −
the Mars pathfinder project. This software shut down at irregulare+ −
intervals leading to loss of project time (the mission and data was+ −
fortunately not lost, because of clever system design). The problem+ −
was that a high priority process and could only be restarted the+ −
next day.+ −
+ −
+ −
Priority inversion referrers to the phenomena where tasks with higher + −
priority are blocked by ones with lower priority. If priority inversion + −
is not controlled, there will be no guarantee the urgent tasks will be + −
processed in time. As reported in \cite{Reeves-Glenn-1998}, + −
priority inversion used to cause software system resets and data lose in + −
JPL's Mars pathfinder project. Therefore, the avoiding, detecting and controlling + −
of priority inversion is a key issue to attain predictability in priority + −
based real-time systems. + −
+ −
The priority inversion phenomenon was first published in \cite{Lampson:Redell:cacm:1980}. + −
The two protocols widely used to eliminate priority inversion, namely + −
PI (Priority Inheritance) and PCE (Priority Ceiling Emulation), were proposed + −
in \cite{journals/tc/ShaRL90}. PCE is less convenient to use because it requires + −
static analysis of programs. Therefore, PI is more commonly used in + −
practice\cite{locke-july02}. However, as pointed out in the literature, + −
the analysis of priority inheritance protocol is quite subtle\cite{yodaiken-july02}. + −
A formal analysis will certainly be helpful for us to understand and correctly + −
implement PI. All existing formal analysis of PI+ −
\cite{conf/fase/JahierHR09,WellingsBSB07,Faria08} are based on the model checking + −
technology. Because of the state explosion problem, model check + −
is much like an exhaustive testing of finite models with limited size. + −
The results obtained can not be safely generalized to models with arbitrarily + −
large size. Worse still, since model checking is fully automatic, it give little + −
insight on why the formal model is correct. It is therefore + −
definitely desirable to analyze PI using theorem proving, which gives + −
more general results as well as deeper insight. And this is the purpose + −
of this paper which gives a formal analysis of PI in the interactive + −
theorem prover Isabelle using Higher Order Logic (HOL). The formalization + −
focuses on on two issues:+ −
+ −
\begin{enumerate}+ −
\item The correctness of the protocol model itself. A series of desirable properties is + −
derived until we are fully convinced that the formal model of PI does + −
eliminate priority inversion. And a better understanding of PI is so obtained + −
in due course. For example, we find through formalization that the choice of + −
next thread to take hold when a + −
resource is released is irrelevant for the very basic property of PI to hold. + −
A point never mentioned in literature. + −
\item The correctness of the implementation. A series of properties is derived the meaning + −
of which can be used as guidelines on how PI can be implemented efficiently and correctly. + −
\end{enumerate} + −
+ −
The rest of the paper is organized as follows: Section \ref{overview} gives an overview + −
of PI. Section \ref{model} introduces the formal model of PI. Section \ref{general} + −
discusses a series of basic properties of PI. Section \ref{extension} shows formally + −
how priority inversion is controlled by PI. Section \ref{implement} gives properties + −
which can be used for guidelines of implementation. Section \ref{related} discusses + −
related works. Section \ref{conclusion} concludes the whole paper.+ −
+ −
+ −
Contributions+ −
+ −
Despite the wide use of Priority Inheritance Protocol in real time operating+ −
system, it's correctness has never been formally proved and mechanically checked. + −
All existing verification are based on model checking technology. Full automatic+ −
verification gives little help to understand why the protocol is correct. + −
And results such obtained only apply to models of limited size. + −
This paper presents a formal verification based on theorem proving. + −
Machine checked formal proof does help to get deeper understanding. We found + −
the fact which is not mentioned in the literature, that the choice of next + −
thread to take over when an critical resource is release does not affect the correctness+ −
of the protocol. The paper also shows how formal proof can help to construct + −
correct and efficient implementation.\bigskip + −
+ −
*}+ −
+ −
section {* An overview of priority inversion and priority inheritance \label{overview} *}+ −
+ −
text {*+ −
+ −
Priority inversion refers to the phenomenon when a thread with high priority is blocked + −
by a thread with low priority. Priority happens when the high priority thread requests + −
for some critical resource already taken by the low priority thread. Since the high + −
priority thread has to wait for the low priority thread to complete, it is said to be + −
blocked by the low priority thread. Priority inversion might prevent high priority + −
thread from fulfill its task in time if the duration of priority inversion is indefinite + −
and unpredictable. Indefinite priority inversion happens when indefinite number + −
of threads with medium priorities is activated during the period when the high + −
priority thread is blocked by the low priority thread. Although these medium + −
priority threads can not preempt the high priority thread directly, they are able + −
to preempt the low priority threads and cause it to stay in critical section for + −
an indefinite long duration. In this way, the high priority thread may be blocked indefinitely. + −
+ −
Priority inheritance is one protocol proposed to avoid indefinite priority inversion. + −
The basic idea is to let the high priority thread donate its priority to the low priority + −
thread holding the critical resource, so that it will not be preempted by medium priority + −
threads. The thread with highest priority will not be blocked unless it is requesting + −
some critical resource already taken by other threads. Viewed from a different angle, + −
any thread which is able to block the highest priority threads must already hold some + −
critical resource. Further more, it must have hold some critical resource at the + −
moment the highest priority is created, otherwise, it may never get change to run and + −
get hold. Since the number of such resource holding lower priority threads is finite, + −
if every one of them finishes with its own critical section in a definite duration, + −
the duration the highest priority thread is blocked is definite as well. The key to + −
guarantee lower priority threads to finish in definite is to donate them the highest + −
priority. In such cases, the lower priority threads is said to have inherited the + −
highest priority. And this explains the name of the protocol: + −
{\em Priority Inheritance} and how Priority Inheritance prevents indefinite delay.+ −
+ −
The objectives of this paper are:+ −
\begin{enumerate}+ −
\item Build the above mentioned idea into formal model and prove a series of properties + −
until we are convinced that the formal model does fulfill the original idea. + −
\item Show how formally derived properties can be used as guidelines for correct + −
and efficient implementation.+ −
\end{enumerate}+ −
The proof is totally formal in the sense that every detail is reduced to the + −
very first principles of Higher Order Logic. The nature of interactive theorem + −
proving is for the human user to persuade computer program to accept its arguments. + −
A clear and simple understanding of the problem at hand is both a prerequisite and a + −
byproduct of such an effort, because everything has finally be reduced to the very + −
first principle to be checked mechanically. The former intuitive explanation of + −
Priority Inheritance is just such a byproduct. + −
*}+ −
+ −
section {* Formal model of Priority Inheritance \label{model} *}+ −
text {*+ −
\input{../../generated/PrioGDef}+ −
*}+ −
+ −
section {* General properties of Priority Inheritance \label{general} *}+ −
(*<*)+ −
ML {*+ −
(*val () = show_question_marks_default := false;*)+ −
*}+ −
(*>*)+ −
+ −
text {*+ −
The following are several very basic prioprites:+ −
\begin{enumerate}+ −
\item All runing threads must be ready (@{text "runing_ready"}):+ −
@{thm[display] "runing_ready"} + −
\item All ready threads must be living (@{text "readys_threads"}):+ −
@{thm[display] "readys_threads"} + −
\item There are finite many living threads at any moment (@{text "finite_threads"}):+ −
@{thm[display] "finite_threads"} + −
\item Every waiting queue does not contain duplcated elements (@{text "wq_distinct"}): + −
@{thm[display] "wq_distinct"} + −
\item All threads in waiting queues are living threads (@{text "wq_threads"}): + −
@{thm[display] "wq_threads"} + −
\item The event which can get a thread into waiting queue must be @{term "P"}-events+ −
(@{text "block_pre"}): + −
@{thm[display] "block_pre"} + −
\item A thread may never wait for two different critical resources+ −
(@{text "waiting_unique"}): + −
@{thm[display] waiting_unique[of _ _ "cs\<^isub>1" "cs\<^isub>2"]}+ −
\item Every resource can only be held by one thread+ −
(@{text "held_unique"}): + −
@{thm[display] held_unique[of _ "th\<^isub>1" _ "th\<^isub>2"]}+ −
\item Every living thread has an unique precedence+ −
(@{text "preced_unique"}): + −
@{thm[display] preced_unique[of "th\<^isub>1" _ "th\<^isub>2"]}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
The following lemmas show how RAG is changed with the execution of events:+ −
\begin{enumerate}+ −
\item Execution of @{term "Set"} does not change RAG (@{text "depend_set_unchanged"}):+ −
@{thm[display] depend_set_unchanged}+ −
\item Execution of @{term "Create"} does not change RAG (@{text "depend_create_unchanged"}):+ −
@{thm[display] depend_create_unchanged}+ −
\item Execution of @{term "Exit"} does not change RAG (@{text "depend_exit_unchanged"}):+ −
@{thm[display] depend_exit_unchanged}+ −
\item Execution of @{term "P"} (@{text "step_depend_p"}):+ −
@{thm[display] step_depend_p}+ −
\item Execution of @{term "V"} (@{text "step_depend_v"}):+ −
@{thm[display] step_depend_v}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
These properties are used to derive the following important results about RAG:+ −
\begin{enumerate}+ −
\item RAG is loop free (@{text "acyclic_depend"}):+ −
@{thm [display] acyclic_depend}+ −
\item RAGs are finite (@{text "finite_depend"}):+ −
@{thm [display] finite_depend}+ −
\item Reverse paths in RAG are well founded (@{text "wf_dep_converse"}):+ −
@{thm [display] wf_dep_converse}+ −
\item The dependence relation represented by RAG has a tree structure (@{text "unique_depend"}):+ −
@{thm [display] unique_depend[of _ _ "n\<^isub>1" "n\<^isub>2"]}+ −
\item All threads in RAG are living threads + −
(@{text "dm_depend_threads"} and @{text "range_in"}):+ −
@{thm [display] dm_depend_threads range_in}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
The following lemmas show how every node in RAG can be chased to ready threads:+ −
\begin{enumerate}+ −
\item Every node in RAG can be chased to a ready thread (@{text "chain_building"}):+ −
@{thm [display] chain_building[rule_format]}+ −
\item The ready thread chased to is unique (@{text "dchain_unique"}):+ −
@{thm [display] dchain_unique[of _ _ "th\<^isub>1" "th\<^isub>2"]}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
Properties about @{term "next_th"}:+ −
\begin{enumerate}+ −
\item The thread taking over is different from the thread which is releasing+ −
(@{text "next_th_neq"}):+ −
@{thm [display] next_th_neq}+ −
\item The thread taking over is unique+ −
(@{text "next_th_unique"}):+ −
@{thm [display] next_th_unique[of _ _ _ "th\<^isub>1" "th\<^isub>2"]} + −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
Some deeper results about the system:+ −
\begin{enumerate}+ −
\item There can only be one running thread (@{text "runing_unique"}):+ −
@{thm [display] runing_unique[of _ "th\<^isub>1" "th\<^isub>2"]}+ −
\item The maximum of @{term "cp"} and @{term "preced"} are equal (@{text "max_cp_eq"}):+ −
@{thm [display] max_cp_eq}+ −
\item There must be one ready thread having the max @{term "cp"}-value + −
(@{text "max_cp_readys_threads"}):+ −
@{thm [display] max_cp_readys_threads}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
The relationship between the count of @{text "P"} and @{text "V"} and the number of + −
critical resources held by a thread is given as follows:+ −
\begin{enumerate}+ −
\item The @{term "V"}-operation decreases the number of critical resources + −
one thread holds (@{text "cntCS_v_dec"})+ −
@{thm [display] cntCS_v_dec}+ −
\item The number of @{text "V"} never exceeds the number of @{text "P"} + −
(@{text "cnp_cnv_cncs"}):+ −
@{thm [display] cnp_cnv_cncs}+ −
\item The number of @{text "V"} equals the number of @{text "P"} when + −
the relevant thread is not living:+ −
(@{text "cnp_cnv_eq"}):+ −
@{thm [display] cnp_cnv_eq}+ −
\item When a thread is not living, it does not hold any critical resource + −
(@{text "not_thread_holdents"}):+ −
@{thm [display] not_thread_holdents}+ −
\item When the number of @{text "P"} equals the number of @{text "V"}, the relevant + −
thread does not hold any critical resource, therefore no thread can depend on it+ −
(@{text "count_eq_dependents"}):+ −
@{thm [display] count_eq_dependents}+ −
\end{enumerate}+ −
*}+ −
+ −
section {* Key properties \label{extension} *}+ −
+ −
(*<*)+ −
context extend_highest_gen+ −
begin+ −
(*>*)+ −
+ −
text {*+ −
The essential of {\em Priority Inheritance} is to avoid indefinite priority inversion. For this + −
purpose, we need to investigate what happens after one thread takes the highest precedence. + −
A locale is used to describe such a situation, which assumes:+ −
\begin{enumerate}+ −
\item @{term "s"} is a valid state (@{text "vt_s"}):+ −
@{thm vt_s}.+ −
\item @{term "th"} is a living thread in @{term "s"} (@{text "threads_s"}):+ −
@{thm threads_s}.+ −
\item @{term "th"} has the highest precedence in @{term "s"} (@{text "highest"}):+ −
@{thm highest}.+ −
\item The precedence of @{term "th"} is @{term "Prc prio tm"} (@{text "preced_th"}):+ −
@{thm preced_th}.+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
Under these assumptions, some basic priority can be derived for @{term "th"}:+ −
\begin{enumerate}+ −
\item The current precedence of @{term "th"} equals its own precedence (@{text "eq_cp_s_th"}):+ −
@{thm [display] eq_cp_s_th}+ −
\item The current precedence of @{term "th"} is the highest precedence in + −
the system (@{text "highest_cp_preced"}):+ −
@{thm [display] highest_cp_preced}+ −
\item The precedence of @{term "th"} is the highest precedence + −
in the system (@{text "highest_preced_thread"}):+ −
@{thm [display] highest_preced_thread}+ −
\item The current precedence of @{term "th"} is the highest current precedence + −
in the system (@{text "highest'"}):+ −
@{thm [display] highest'}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
To analysis what happens after state @{term "s"} a sub-locale is defined, which + −
assumes:+ −
\begin{enumerate}+ −
\item @{term "t"} is a valid extension of @{term "s"} (@{text "vt_t"}): @{thm vt_t}.+ −
\item Any thread created in @{term "t"} has priority no higher than @{term "prio"}, therefore+ −
its precedence can not be higher than @{term "th"}, therefore+ −
@{term "th"} remain to be the one with the highest precedence+ −
(@{text "create_low"}):+ −
@{thm [display] create_low}+ −
\item Any adjustment of priority in + −
@{term "t"} does not happen to @{term "th"} and + −
the priority set is no higher than @{term "prio"}, therefore+ −
@{term "th"} remain to be the one with the highest precedence (@{text "set_diff_low"}):+ −
@{thm [display] set_diff_low}+ −
\item Since we are investigating what happens to @{term "th"}, it is assumed + −
@{term "th"} does not exit during @{term "t"} (@{text "exit_diff"}):+ −
@{thm [display] exit_diff}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
All these assumptions are put into a predicate @{term "extend_highest_gen"}. + −
It can be proved that @{term "extend_highest_gen"} holds + −
for any moment @{text "i"} in it @{term "t"} (@{text "red_moment"}):+ −
@{thm [display] red_moment}+ −
+ −
From this, an induction principle can be derived for @{text "t"}, so that + −
properties already derived for @{term "t"} can be applied to any prefix + −
of @{text "t"} in the proof of new properties + −
about @{term "t"} (@{text "ind"}):+ −
\begin{center}+ −
@{thm[display] ind}+ −
\end{center}+ −
+ −
The following properties can be proved about @{term "th"} in @{term "t"}:+ −
\begin{enumerate}+ −
\item In @{term "t"}, thread @{term "th"} is kept live and its + −
precedence is preserved as well+ −
(@{text "th_kept"}): + −
@{thm [display] th_kept}+ −
\item In @{term "t"}, thread @{term "th"}'s precedence is always the maximum among + −
all living threads+ −
(@{text "max_preced"}): + −
@{thm [display] max_preced}+ −
\item In @{term "t"}, thread @{term "th"}'s current precedence is always the maximum precedence+ −
among all living threads+ −
(@{text "th_cp_max_preced"}): + −
@{thm [display] th_cp_max_preced}+ −
\item In @{term "t"}, thread @{term "th"}'s current precedence is always the maximum current + −
precedence among all living threads+ −
(@{text "th_cp_max"}): + −
@{thm [display] th_cp_max}+ −
\item In @{term "t"}, thread @{term "th"}'s current precedence equals its precedence at moment + −
@{term "s"}+ −
(@{text "th_cp_preced"}): + −
@{thm [display] th_cp_preced}+ −
\end{enumerate}+ −
*}+ −
+ −
text {* \noindent+ −
The main theorem is to characterizing the running thread during @{term "t"} + −
(@{text "runing_inversion_2"}):+ −
@{thm [display] runing_inversion_2}+ −
According to this, if a thread is running, it is either @{term "th"} or was+ −
already live and held some resource + −
at moment @{text "s"} (expressed by: @{text "cntV s th' < cntP s th'"}).+ −
+ −
Since there are only finite many threads live and holding some resource at any moment,+ −
if every such thread can release all its resources in finite duration, then after finite+ −
duration, none of them may block @{term "th"} anymore. So, no priority inversion may happen+ −
then.+ −
*}+ −
+ −
(*<*)+ −
end+ −
(*>*)+ −
+ −
section {* Properties to guide implementation \label{implement} *}+ −
+ −
text {*+ −
The properties (especially @{text "runing_inversion_2"}) convinced us that model defined + −
in section \ref{model} does prevent indefinite priority inversion and therefore fulfills + −
the fundamental requirement of Priority Inheritance protocol. Another purpose of this paper + −
is to show how this model can be used to guide a concrete implementation.+ −
*}+ −
+ −
+ −
section {* Related works \label{related} *}+ −
+ −
text {*+ −
\begin{enumerate}+ −
\item {\em Integrating Priority Inheritance Algorithms in the Real-Time Specification for Java}+ −
\cite{WellingsBSB07} models and verifies the combination of Priority Inheritance (PI) and + −
Priority Ceiling Emulation (PCE) protocols in the setting of Java virtual machine + −
using extended Timed Automata(TA) formalism of the UPPAAL tool. Although a detailed + −
formal model of combined PI and PCE is given, the number of properties is quite + −
small and the focus is put on the harmonious working of PI and PCE. Most key features of PI + −
(as well as PCE) are not shown. Because of the limitation of the model checking technique+ −
used there, properties are shown only for a small number of scenarios. Therefore, + −
the verification does not show the correctness of the formal model itself in a + −
convincing way. + −
\item {\em Formal Development of Solutions for Real-Time Operating Systems with TLA+/TLC}+ −
\cite{Faria08}. A formal model of PI is given in TLA+. Only 3 properties are shown + −
for PI using model checking. The limitation of model checking is intrinsic to the work.+ −
\item {\em Synchronous modeling and validation of priority inheritance schedulers}+ −
\cite{conf/fase/JahierHR09}. Gives a formal model+ −
of PI and PCE in AADL (Architecture Analysis \& Design Language) and checked + −
several properties using model checking. The number of properties shown there is + −
less than here and the scale is also limited by the model checking technique. + −
\item {\em The Priority Ceiling Protocol: Formalization and Analysis Using PVS}+ −
\cite{dutertre99b}. Formalized another protocol for Priority Inversion in the + −
interactive theorem proving system PVS.+ −
\end{enumerate}+ −
+ −
+ −
There are several works on inversion avoidance:+ −
\begin{enumerate}+ −
\item {\em Solving the group priority inversion problem in a timed asynchronous system}+ −
\cite{Wang:2002:SGP}. The notion of Group Priority Inversion is introduced. The main + −
strategy is still inversion avoidance. The method is by reordering requests + −
in the setting of Client-Server.+ −
\item {\em A Formalization of Priority Inversion} \cite{journals/rts/BabaogluMS93}. + −
Formalized the notion of Priority + −
Inversion and proposes methods to avoid it. + −
\end{enumerate}+ −
+ −
{\em Examples of inaccurate specification of the protocol ???}.+ −
+ −
*}+ −
+ −
section {* Conclusions \label{conclusion} *}+ −
+ −
(*<*)+ −
end+ −
(*>*)+ −