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Verification of mutual exclusion algorithms with EST

ERK 2001. Verification of mutual exclusion algorithms with EST. Robert Meolic , Tatjana Kapus, Ernest Gungl, Zmago Brezočnik. Verification of mutual exclusion algorithms with EST. Introduction Mutual exclusion algorithms Formal verification Equivalence checking Model checking

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Verification of mutual exclusion algorithms with EST

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  1. ERK 2001 Verification of mutual exclusion algorithms with EST Robert Meolic, Tatjana Kapus, Ernest Gungl, Zmago Brezočnik

  2. Verification of mutual exclusion algorithms with EST • Introduction • Mutual exclusion algorithms • Formal verification • Equivalence checking • Model checking • Demonstration

  3. Introduction • A critical section is a block of code in a thread that accesses one or more shared variables in a read-update-write fashion. • In such a situation we want mutual exclusion: only one thread at a time can access (read-update-write) a shared variable at a time. • The mutual exclusion problem is how to keep two or more threads from being in their critical sections at the same time.

  4. A critical section and mutual exclusion while (true) { outsideCS(); wantToEnterCS(i); insideCS(); finishedInCS(i); }

  5. Dekker’s algorithm (1965) while true dobegin<noncritical section>;bi := true;whilebjdo beginif k = j then begin bi := false;whilek = j do skip; bi := true;end;end;<critical section>; k := j;bi := false;end Mutual exclusion with shared variables and referee: • variables b1 and b2 (false or true) • variable k (1 or 2) Dekker’s algorithm is very efficientalgorithm for handling twoconcurrentprocesses.

  6. Hyman’s algorithm (1966) while true dobegin<noncritical section>;bi := true;while k  i do beginwhile bj do skip;k := i;end;<critical section>;bi := false;end

  7. Peterson’s algorithm (1981) while true dobegin<noncritical section>;bi := true;k := j;while (bj and k = j) do skip;<critical section>;bi := false;end

  8. Formal verification • Mentor Graphics: FormalPro • Equivalence checking Have I corrupted my design with changes? • Model checking Have I designed what I wanted to design?

  9. Why formal verification? Source: Mentor Graphics Web page

  10. Source: Mentor Graphics

  11. Efficient Symbolic Tools • Realized on Faculty of Electrical Engineering and Computer Science in Maribor • The project started in year 1992 • GNU General Public License • It runs on many different computers with different operating systems, including HP-UX, Linux, and Windows. http://www.el.feri.uni-mb.si/est/

  12. Demonstration • Verification of Hyman’s algorithm (MC)- does not preserve mutual exclusion • Verification of Peterson’s algorithm (MC)- preserve mutual exclusion • Our model contains divergent states (MC)- we eliminate divergent states • Modification was not correct!(EC) - we improve our model

  13. b2rf? Modified model of Peterson’s algorithm  =

  14. Further work • Mutual exclusion algorithms for 3 or more processes, e.g. Lamport’s algorithm • Automatic trasformation from programming language to process algebra description • Model checking with fairness constraints

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