Token1 1 f t 3 enter critical section criticalsection

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token[1] = 1 F && T //3. Enter Critical section critical_section(); T && F //3. Enter Critical section critical_section();
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10 Bakery Algorithm Bakery Algorithm while (TRUE) { //1. Receive a token choosing[OwnID] = true; token[OwnID]= max(token[0],token[1],..,token[n-1])+1; choosing[OwnID] = false; //2. Wait for turn for (OthersID = 0;OthersID<n;OthersID++) while(choosing[OthersID]); while (token[OthersID]!=0 &&(token[OthersID],OthersID)< (token[OwnID],OwnID)); //3. Enter Critical section critical_section(); //4. Leave Critical Section token[OwnID] = 0; }
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11 Bakery Algorithm Bakery Algorithm The reason for choosing is to prevent the second while loop being entered when process P OthersID is setting its token[OthersID] . choosing[OthersID] is true if P OthersID is choosing a token. If a process P OthersID is choosing a token when Pi tries to look at it, Pi waits until P OthersID has done so before looking
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12 Hardware Solutions Hardware Solutions Disabling/Enabling Interrupts Fetch Next Instruction Execute Instruction Check for Interrupt: Process Interrupt Start Halt Decode Instruction No Interrupt Checking No Context Switching This solution is dangerous It will not work if we have multiple CPUs Since, the Disable Interrupt instruction will execute on only one CPU Rest of the CPUs, will continue checking the interrupts
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13 Software Lock Software Lock Process 1 1.while (FLAG == FALSE); 2.FLAG = FALSE; 4.FLAG = TRUE; 5.noncritical_section(); Process 2 1.while (FLAG == FALSE); 2.
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  • Fall '09
  • ABAD
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