Deadlocks Detection and Avoidance. Prof. Sirer CS 4410 Cornell University

Transcription

1 Deadlocks Detectio ad Avoidace Prof. Sirer CS 4410 Corell Uiversity

2 System Model There are o-shared computer resources Maybe more tha oe istace Priters, Semaphores, Tape drives, CPU Processes eed access to these resources Acquire resource w If resource is available, access is grated w If ot available, the process is blocked Use resource Release resource Udesirable sceario: Process A acquires resource 1, ad is waitig for resource 2 Process B acquires resource 2, ad is waitig for resource 1 Þ Deadlock!

3 Example 1: Semaphores semaphore: mutex1 = 1 /* protects resource 1 */ mutex2 = 1 /* protects resource 2 */ Process A code: { /* iitial compute */ P(file) P(priter) /* use both resources */ Process B code: { /* iitial compute */ P(priter) P(file) /* use both resources */ } V(priter) V(file) } V(file) V(priter)

4 Simplest deadlock

5 Example 2: Diig Philosophers class Philosopher: chopsticks[n] = [Semaphore(1), ] Def iit (myum) self.id = myum Def eat(): right = (self.id+1) % N left = (self.id-1+n) % N while True: # om om om Philosophers go out for Chiese food They eed exclusive access to two chopsticks to eat their food

6 Example 2: Diig Philosophers Philosophers go out for Chiese food class Philosopher: chopsticks[n] = [Semaphore(1), ] Def iit (myum) self.id = myum Def eat(): right = (self.id+1) % N left = (self.id-1+n) % N while True: P(left) P(right) # om om om V(right) V(left) They eed exclusive access to two chopsticks to eat their food

7 More Complicated Deadlock

8 Deadlocks Deadlock exists amog a set of processes if Every process is waitig for a evet This evet ca be caused oly by aother process i the set that i tur is waitig for a evet Typically, the evet is the acquire or release of aother resource Kasas 20th cetury law: Whe two trais approach each other at a crossig, both shall come to a full stop ad either shall start up agai util the other has goe

9 Four Coditios for Deadlock Necessary coditios for deadlock to exist: Mutual Exclusio w At least oe resource must be held i o-sharable mode Hold ad wait w There exists a process holdig a resource, ad waitig for aother No preemptio w Resources caot be preempted Circular wait w There exists a set of processes {P 1, P 2, P N }, such that P 1 is waitig for P 2, P 2 for P 3,. ad P N for P 1 All four coditios must hold for deadlock to occur

10 Real World Deadlocks? Truck A has to wait for truck B to move Not deadlocked

11 Real World Deadlocks? Gridlock

12 Deadlock i Real Life?

13 Deadlock i Real Life? No circular wait! Not a deadlock! At least, ot as far as we ca see from the picture Will ultimately resolve itself give eough time

14 Deadlock i Real Life

15 Deadlock Prevetio

16 Deadlock Prevetio Ca the OS prevet deadlocks? Prevetio: Negate oe of ecessary coditios Mutual exclusio: w Make resources sharable w Not always possible (priters?) Hold ad wait w Do ot hold resources whe waitig for aother Þ Request all resources before begiig executio Processes do ot kow what all they will eed Starvatio (if waitig o may popular resources) Low utilizatio (Need resource oly for a bit) w Alterative: Release all resources before requestig aythig ew Still has the last two problems

17 Deadlock Prevetio Prevetio: Negate oe of ecessary coditios No preemptio: w Make resources preemptable (2 approaches) Preempt requestig processes resources if all ot available Preempt resources of waitig processes to satisfy request w Good whe easy to save ad restore state of resource CPU registers, memory virtualizatio Circular wait: (2 approaches) w Sigle lock for etire system? (Problems) w Impose partial orderig o resources, request them i order

18 Deadlock Prevetio 2 Prevetio: Breakig circular wait Order resources (lock1, lock2, ) Acquire resources i strictly icreasig/decreasig order Whe requests to multiple resources of same order: w Make the request a sigle operatio Ituitio: Cycle requires a edge from low to high, ad from high to low umbered ode, or to same ode Orderig ot always possible, low resource utilizatio