CRITICAL SECTION

1.

// functions to enter/leave the // critical section void critical_section_enter_t0() { while (turn != T0) continue; } void critical_section_leave_t0() { turn = T1; }

// functions void critical_section_enter_t1() { while (turn != T1) continue; } void critical_section_leave_t1() { turn = T0; }

i. Does this proposal provide mutual exclusion for the critical
section? Justify your answer!
Yes, it does. The variable ‘turn’ is either T0 or T1, and if it is T0, then
only thread T0 can progress past ‘critical_section_enter_t0’, and vice
versa.

ii. Is this proposal a satisfactory solution to the critical section
problem? Justify your answer!

No, it is not. It does not guarantee progress. In particular, thread T0
cannot reenter the critical section after it has left it unless thread T1
entered and left the critical section in the interim since critical_section_leave_t0() sets turn = T1. Only critical_section_leave_t1() can change it back to T0 again.

Consider:
critical_section_enter_t0();
critical_section_leave_t0();
// may block even though t1 is not in the critical section
critical_section_enter_t0();
critical_section_leave_t0();

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http://pages.cs.wisc.edu/~bart/537/quizzes/quiz2.html