Verification of mutual exclusion algorithms with EST

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Verification of mutual exclusion algorithms with
EST
ERK 2001

• Robert Meolic, Tatjana Kapus, Ernest Gungl,
  Zmago Brezocnik

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Verification of mutual exclusion algorithms with
EST

• Introduction
• Mutual exclusion algorithms
• Formal verification
• Equivalence checking
• Model checking
• Demonstration

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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.

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A critical section and mutual exclusion

• while (true)
• outsideCS()
• wantToEnterCS(i)
• insideCS()
• finishedInCS(i)

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Dekkers algorithm (1965)
while true dobegin ltnoncritical sectiongt
bi true while bj do begin if k
j then begin bi false
while k j do skip bi true
end end ltcritical sectiongt k
j bi falseend

• Mutual exclusion with shared variables and
  referee
• variables b1 and b2 (false or true)
• variable k (1 or 2)

Dekkers algorithm is very efficient algorithm
for handling two concurrent processes.
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Hymans algorithm (1966)
 
while true dobegin ltnoncritical sectiongt
bi true while k ? i do begin
while bj do skip k i end
ltcritical sectiongt bi falseend
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Petersons algorithm (1981)
while true dobegin ltnoncritical sectiongt
bi true k j while (bj and k j)
do skip ltcritical sectiongt bi
falseend
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Formal verification

• Mentor Graphics FormalPro
• Equivalence checking
• Have I corrupted my design with changes?
• Model checking
• Have I designed what I wanted to design?

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Why formal verification?
Source Mentor Graphics Web page
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Source Mentor Graphics
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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/
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Demonstration

• Verification of Hymans algorithm (MC) - does
  not preserve mutual exclusion
• Verification of Petersons algorithm (MC) -
  preserve mutual exclusion
• Our model contains divergent states (MC) - we
  eliminate divergent states
• Modification was not correct! (EC) - we improve
  our model

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Modified model of Petersons algorithm
 
?

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Further work

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