CS162!Operating Systems and!Systems Programming!Lecture 7!Deadlock, CPU Scheduling"February 9, 2011!Ion Stoica!http://inst.eecs.berkeley.edu/~cs162!Lec 7.2!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Read/Writer Revisited"Reader() { // check into system lock.Acquire(); while ((AW + WW) > 0) { WR++; okToRead.wait(&lock); WR--; } AR++; lock.release(); // read-only access AccessDbase(ReadOnly); // check out of system lock.Acquire(); AR--; if (AR == 0 && WW > 0) okToWrite.signal(); lock.Release(); }!Writer() { // check into system lock.Acquire(); while ((AW + AR) > 0) { WW++; okToWrite.wait(&lock); WW--; } AW++; lock.release(); // read/write access AccessDbase(ReadWrite); // check out of system lock.Acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okToRead.broadcast(); } lock.Release(); }!What if we remove this line?!Lec 7.3!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Read/Writer Revisited"Reader() { // check into system lock.Acquire(); while ((AW + WW) > 0) { WR++; okToRead.wait(&lock); WR--; } AR++; lock.release(); // read-only access AccessDbase(ReadOnly); // check out of system lock.Acquire(); AR--; if (AR == 0 && WW > 0) okToWrite.broadcast(); lock.Release(); }!Writer() { // check into system lock.Acquire(); while ((AW + AR) > 0) { WW++; okToWrite.wait(&lock); WW--; } AW++; lock.release(); // read/write access AccessDbase(ReadWrite); // check out of system lock.Acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okToRead.broadcast(); } lock.Release(); }!What if we turn signal to broadcast?!Lec 7.4!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Read/Writer Revisited"Reader() { // check into system lock.Acquire(); while ((AW + WW) > 0) { WR++; okContinue.wait(&lock); WR--; } AR++; lock.release(); // read-only access AccessDbase(ReadOnly); // check out of system lock.Acquire(); AR--; if (AR == 0 && WW > 0) okContinue.signal(); lock.Release(); }!Writer() { // check into system lock.Acquire(); while ((AW + AR) > 0) { WW++; okContinue.wait(&lock); WW--; } AW++; lock.release(); // read/write access AccessDbase(ReadWrite); // check out of system lock.Acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okContinue.broadcast(); } lock.Release(); }!What if we turn okToWrite and okToRead into okContinue?!Lec 7.5!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Read/Writer Revisited"Reader() { // check into system lock.Acquire(); while ((AW + WW) > 0) { WR++; okContinue.wait(&lock); WR--; } AR++; lock.release(); // read-only access AccessDbase(ReadOnly); // check out of system lock.Acquire(); AR--; if (AR == 0 && WW > 0) okContinue.signal(); lock.Release(); }!Writer() { // check into system lock.Acquire(); while ((AW + AR) > 0) { WW++; okContinue.wait(&lock); WW--; } AW++; lock.release(); // read/write access AccessDbase(ReadWrite); // check out of system lock.Acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okContinue.broadcast(); } lock.Release(); }!• R1 arrives !• W1, R2 arrive while R1 reads!• R1 signals R2!Lec 7.6!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Read/Writer Revisited"Reader() { // check into system lock.Acquire(); while ((AW + WW) > 0) { WR++; okContinue.wait(&lock); WR--; } AR++; lock.release(); // read-only access AccessDbase(ReadOnly); // check out of system lock.Acquire(); AR--; if (AR == 0 && WW > 0) okContinue.broadcast(); lock.Release(); }!Writer() { // check into system lock.Acquire(); while ((AW + AR) > 0) { WW++; okContinue.wait(&lock); WW--; } AW++; lock.release(); // read/write access AccessDbase(ReadWrite); // check out of system lock.Acquire(); AW--; if (WW > 0){ okToWrite.signal(); } else if (WR > 0) { okContinue.broadcast(); } lock.Release(); }!Need to change to broadcast!!Why?!Lec 7.7!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!• Resources – passive entities needed by threads to do their work!– CPU time, disk space, memory!• Two types of resources:!– Preemptable – can take it away!» CPU, Embedded security chip!– Non-preemptable – must leave it with the thread!» Disk space, printer, chunk of virtual address space!» Critical section !• Resources may require exclusive access or may be sharable!– Read-only files are typically sharable!– Printers are not sharable during time of printing!• One of the major tasks of an operating system is to manage resources!Resources"Lec 7.8!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Starvation vs Deadlock"• Starvation vs. Deadlock!– Starvation: thread waits indefinitely!» Example, low-priority thread waiting for resources constantly in use by high-priority threads!– Deadlock: circular waiting for resources!» Thread A owns Res 1 and is waiting for Res 2#Thread B owns Res 2 and is waiting for Res 1!– Deadlock ⇒ Starvation but not vice versa!» Starvation can end (but doesnʼt have to)!» Deadlock canʼt end without external intervention!Res 2 Res 1 Thread B Thread A Wait For Wait For Owned By Owned ByLec 7.9!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Conditions for Deadlock"• Deadlock not always deterministic – Example 2 mutexes:!! !Thread A Thread B x.P(); y.P(); y.P(); x.P(); y.V(); x.V(); x.V(); y.V(); – Deadlock wonʼt always happen with this code!» Have to have exactly the right timing (“wrong” timing?)!» So you release a piece of software, and you tested it, and there it is, controlling a nuclear power plant…!• Deadlocks occur with multiple resources!– Means you canʼt decompose the problem!– Canʼt solve deadlock for each resource independently!• Example: System with 2 disk drives and two threads!– Each thread needs 2 disk drives to function!– Each thread gets one disk and waits for another one!A: x.P(); B: y.P(); A: y.p(); B: x.P(); ... Deadlock!Lec 7.10!2/9/11! Ion Stoica CS162 ©UCB Spring 2011!Bridge Crossing Example"• Each segment of road can be viewed as a resource!– Car must own the segment
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