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CIS 720

CIS 720. Mutual Exclusion. Critical Section problem. Process i do (true) entry protocol; critical section; exit protocol; non-critical section od. Correctness. Mutual exclusion: at most one process at a time is executing its critical section

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CIS 720

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  1. CIS 720 Mutual Exclusion

  2. Critical Section problem • Process i do (true) entry protocol; critical section; exit protocol; non-critical section od

  3. Correctness • Mutual exclusion: at most one process at a time is executing its critical section • Absence of deadlock: If two or more processes are trying to enter their critical section, at least one will succeed • Absence of unnecessary delay: If a process is trying to enter its critical section and the other processes are executing their non-critical sections or have terminated then the first process is not prevented from entering its critical section. • Eventual entry: A process that is attempting to enter its critical section will eventually succeed.

  4. Invariant based approach CS1 CS2 do (true) do (true)  entry protocol; entry protocol; in1 = true in2 = true critical section critical section exit protocol; exit protocol; in1 = false in2 = false non-critical section non-critical section od

  5. Invariant based approach CS1 CS2 do (true) do (true)  <await (!lock)  lock = true> <await(!lock)  lock = true> critical section critical section lock = false lock = false non-critical section non-critical section od

  6. Test and set instruction CS1 CS2 do (true) do (true)  while (TS(lock)) skip; while(TS(lock)); critical section critical section lock = false lock = false non-critical section non-critical section od

  7. Implementing await statements • CSenter while (!B) { CSexit; CSenter } S; CSexit

  8. Tie Breaker Algorithm in1 = false; in2 = false; last = 1 co CS1: CS2: do true  do true  int = true; last = 1; in2 = true; last = 2; while(in2 /\ last == 1); while(in1 /\ last == 2); critical section critical section in1 = false; in2 = false; non-critical section non-critical section od od oc

  9. Tie Breaker Algorithm in1 = false; in2 = false; last = 1 co CS1: CS2: do true  do true  last = 1; in1 = true; last = 2; in2 = true <await (!in2 \/ last == 2)>; <await (!in1 \/ last == 1)>; critical section critical section in1 = false; in2 = false; non-critical section non-critical section od od oc

  10. Ticket Algorithm next = 1; number = 1; turn[1..n]= [0…0]; co CSi: do true  < turn[i] = number; number = number + 1 > await( turn[i] ) == next critical section <next = next + 1> non-critical section od oc Invariant: next > 0 and for all i, cs[i] in CS  turn[i] == next /\ turn[i] != turn[j] for all j

  11. Bakery algorithm turn1 = 0; turn2 = 0; co CS1: CS2: do true  do true  turn1 = turn2 + 1; turn2 = turn1 + 1 while(turn2 != 0 /\ turn1 > turn2); while(turn1 != 0 and /\ turn2 > turn1); critical section critical section turn1 = 0; turn2 = 0; non-critical section non-critical section od od oc

  12. Bakery algorithm turn1 = 0; turn2 = 0; co CS1: CS2: do true  do true  turn = 1; turn2 = 1 turn1 = turn2 + 1; turn2 = turn1 + 1 while(turn2 != 0 /\ while(turn1 != 0 and /\ (turn1,1) > (turn2,2)); (turn2,2) > (turn1,1)); critical section critical section turn1 = 0; turn2 = 0; non-critical section non-critical section od od oc

  13. Barrier synchronization Worker[i]: do true  code for task i wait for all tasks to complete od

  14. Barrier synchronization Worker[i]: do true  code for task i <count = count + 1> < await( count == n) > od

  15. Barrier synchronization Worker[i]: do true  code for task i <count = count + 1> < await( count == n) > od

  16. Barrier synchronization co worker[i]: Coordinator do true  do true  code for task I; for (i = 1 to n) arrive[i] = 1 await(arrive[i]= 1); await(continue ==1) continue = 1 od od oc

  17. Barrier synchronization co worker[i]: Coordinator do true  do true  code for task I; for (i = 1 to n) arrive[i] = 1 { await(arrive[i]= 1); await(continue[i]==1) arrive[i] = 0; } continue[i] = 0; for (i = 1 to n) continue[i] = 1 od od oc Flag rule: A process that waits for the synchronization flags should reset it.

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