Test and Set Solution

This algorithm solves the critical section problem for n processes using a Test and Set instruction (called TaS here). This instruction does the following function atomically:

        function TaS(var Lock: boolean): boolean;
            TaS := Lock;
            Lock := true;

The solution is:

 1 var waiting: shared array [0..n-1] of boolean;
 2     Lock: shared boolean;
 3     j: 0..n-1;
 4     key: boolean;
 5     ...
 6 repeat      (* process Pi *)
 7     waiting[i] := true;
 8     key := true;
 9     while waiting[i] and key do
10         key := TaS(Lock);
11     waiting[i] := false;
12     (* critical section goes here *)
13     j := i + 1 mod n;
14     while (j <> i) and not waiting[j] do
15         j := j + 1 mod n;
16     if j = i then
17         Lock := false
18     else
19         waiting[j] := false;
20 until false;

lines 1–2: These are global to all processes, and are all initialized to false.

lines 3—4: These are local to each process i and are uninitialized.

lines 7–11: This is the entry section. Basically, waiting[i] is true as long as process i is trying to get into its critical section; if any other process is in that section, then Lock will also be true, and process i will loop in lines 9–10. Once process i can go on, it is no longer waiting for permission to enter, and sets waiting[i] to false (line 11); it then proceeds into the critical section. Note that Lock is set to true by the TaS instruction in line 10 that returns false.

lines 13–19: This is the exit section. When process i leaves the critical section, it must choose which other waiting process may enter next. It starts with the process with the next higher index (line 13). It checks each process to see if that process is waiting for access (lines 14–15); if no-one is, it simply releases the lock (by setting Lock to false; lines 16–17). However, if some other process process j is waiting for entry, process i simply shanges waiting[j] to false to allow process j to enter the critical section (lines 18–19).

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Matt Bishop
Office: 2209 Watershed Sciences
Phone: +1 (530) 752-8060
Email: mabishop@ucdavis.edu
ECS 150, Operating Systems
Version of April 11, 2022 at 10:33PM

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