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cs162_fa08_mt1-2 - Problem 1e[3pts Above we show the...

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CS 162 Fall 2008 Midterm Exam I October 15, 2008 Page 4/21 Reader() { //First check self into system lock.acquire(); while ((AW + WW) > 0) { WR++; okToRead.wait(&lock); WR--; } AR++; lock.release(); // Perform actual read-only access AccessDatabase(ReadOnly); // Now, check out of system lock.acquire(); AR--; if (AR == 0 && WW > 0) okToWrite.signal(); lock.release(); } Writer() { // First check self into system lock.acquire(); while ((AW + AR) > 0) { WW++; okToWrite.wait(&lock); WW--; } AW++; lock.release(); // Perform actual read/write access AccessDatabase(ReadWrite); // Now, check out of system lock.acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okToRead.broadcast(); } lock.release();
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Unformatted text preview: } Problem 1e[3pts]: Above, we show the Readers-Writers example given in class. It used two condition variables, one for waiting readers and one for waiting writers. Suppose that all of the following requests arrive in very short order (while R 1 and R 2 are still executing): Incoming stream: R 1 R 2 W 1 W 2 R 3 R 4 R 5 W 3 R 6 W 4 W 5 R 7 R 8 W 6 R 9 In what order would the above code process the above requests? If you have a group of requests that are equivalent (unordered), indicate this clearly by surrounding them with braces ‘{}’. You can assume that the wait queues for condition variables are FIFO in nature (i.e. signal() wakes up the oldest thread on the queue). Explain how you got your answer....
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