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CIS 720

CIS 720. Correctness of Concurrent Programs. i := 1 max := A[1] do i < n  i = i + 1 if (max < A[i]) max = A[i] od. Concurrency rule.

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CIS 720

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  1. CIS 720 Correctness of Concurrent Programs

  2. i := 1 max := A[1] do i < n  i = i + 1 if (max < A[i]) max = A[i] od

  3. Concurrency rule for all i, { Pi } Si { Qi } { P1 /\ …. /\ Pn} co S1 // …. // Snoc { Q1 /\ …. /\ Qn}

  4. x = 0; y = 1; x = 0; co co x = 1 x = x + 1 // // y = y + 1 x = x + 2 oc oc

  5. Interference freedom • Let abe a statement and C be an assertion. • NI(a, C) iff { pre(a) /\ C } a { C } • Execution of a does not invalidates (or interferes) with C.

  6. { Pi } Si { Qi }, for all I, are interference free if • For all i for all assertions C in proof outline of Si, for all actions a in Sj, i != j NI(a, C) holds

  7. Concurrency rule { Pi } Si { Qi } are interference free { P1 /\ …. /\ Pn} co S1 // …. // Snoc { Q1 /\ …. /\ Qn}

  8. x= 0; co x = x + 1 // x = x + 2 od

  9. Bank transactions co // Transfer(x,y,am): Auditor: ac[x] = ac[x] – am; total = 0; i = 0; ac[y] = ac[y] + am do i < n total = total + ac[i] i = i + 1 od oc

  10. Bank transactions co // Transfer(x,y,am): Auditor: if < !auditon  ac[x] = ac[x] – am; auditon = 1; total = 0; i = 0; ac[y] = ac[y] + am > [] do i < n <auditon && ( (x < i && y < i) ||(x > I && y > i)) total = total + ac[i]  ac[x] = ac[x] – am; ac[y] = ac[y] + am> i = i + 1 fi od auditon = 0 oc

  11. Techniques to avoid interference • Disjoint variables • If the write set of each process is disjoint from the read and write set of other processes, then the proof outlines will be interference free.

  12. Avoiding interference • Weakened assertions x= 0; co x = x + 1 // x = x + 2 od

  13. Avoiding interference • Global Invariants: Assume that every assertion in the proof outline of Pi is of the form I /\ L, where L refers to local variables of Pi or to variables that only Pi writes. - Then the proof outlines will be interference free.

  14. Concurrency rule for all i, { I} Si { I } { I} co S1 // …. // Sn { I}

  15. Avoiding interference • Synchronization

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