CS 153 Design of Operating Systems Spring 18

Transcription

1 CS 153 Design of Operating Systems Spring 18 Lectre 12: Deadlock Instrctor: Chengy Song Slide contribtions from Nael Ab-Ghazaleh, Harsha Madhyvasta and Zhiyn Qian

2 Deadlock the deadly embrace! Synchronization we can easily shoot orselves in the foot Incorrect se of synchronization can block all processes Yo have likely been intitively avoiding this sitation already Consider: processes that se mltiple critical sections/need different resorces If one process tries to access a resorce that a second process holds, and vice-versa, they can never make progress We call this sitation deadlock, and we ll look at: Definition and conditions necessary for deadlock Representation of deadlock conditions Approaches to dealing with deadlock CS 153 Lectre 13 Deadlock 2

3 Deadlock Definition Deadlock is a problem that can arise: When processes compete for access to limited resorces When processes are incorrectly synchronized Definition: Deadlock exists among a set of processes if every process is waiting for an event that can be cased only by another process in the set Process 1 Process 2 locka->acqire(); lockb->acqire(); lockb->acqire(); locka->acqire(); CS 153 Lectre 13 Deadlock 3

4 Real example! 4

5 Conditions for Deadlock Deadlock can exist if and only if the following for conditions hold simltaneosly: 1. Mtal exclsion At least one resorce mst be held in a non-sharable mode 2. Hold and wait There mst be one process holding one resorce and waiting for another resorce 3. No preemption Resorces cannot be preempted (critical sections cannot be aborted externally) 4. Circlar wait There mst exist a set of processes [P 1, P 2, P 3,,P n ] sch that P 1 is waiting for P 2, P 2 for P 3, etc. CS 153 Lectre 13 Deadlock 7

6 Dining Lawyers Each lawyer needs two chopsticks to eat. Each grabs chopstick on the right first. CS 153 Lectre 13 Deadlock 8

7 Lets get formal for a minte Deadlock can be described sing a resorce allocation graph (RAG) The RAG consists of a set of vertices P={P 1, P 2,, P n } of processes and R={R 1, R 2,, R m } of resorces A directed edge from a process to a resorce, P i àr i, means that P i has reqested R j A directed edge from a resorce to a process, R i àp i, means that R j has been allocated to P i Each resorce has a fixed nmber of nits If the graph has no cycles, deadlock cannot exist If the graph has a cycle, deadlock may exist CS 153 Lectre 13 Deadlock 9

8 RAG Example P 1 P 1 R 3 R 1 R 1 R 3 P 2 P 2 P 3 R 2 P 3 R 2 P 4 A cycle and deadlock! Same cycle bt no deadlock. Why? CS 153 Lectre 13 Deadlock 10

9 A Simpler Case If all resorces are single nit and all processes make single reqests, then we can represent the resorce state with a simpler waits-for graph (WFG) The WFG consists of a set of vertices P={P 1, P 2,, P n } of processes A directed edge P i àp j means that P i has reqested a resorce that P j crrently holds If the graph has no cycles, deadlock cannot exist If the graph has a cycle, deadlock exists CS 153 Lectre 13 Deadlock 11

10 Dealing with Deadlock There are for approaches for dealing with deadlock: Ignore it how lcky do yo feel? Prevention make it impossible for deadlock to happen Avoidance control allocation of resorces Detection and Recovery look for a cycle in dependencies CS 153 Lectre 13 Deadlock 12

11 Deadlock Prevention Prevention Ensre that at least one of the necessary conditions cannot happen Mtal exclsion» Make resorces sharable (not generally practical) Hold and wait» Process cannot hold one resorce when reqesting another Preemption» OS can preempt resorce (costly) Circlar wait» Impose an ordering (nmbering) on the resorces and reqest them in order (poplar implementation techniqe) CS 153 Lectre 13 Deadlock 13

12 Deadlock Prevention One shot allocation: ask for all yor resorces in one shot; no more resorces can be reqested What ingredient does this prevent? Comments? Preemption Nice: Give p a resorce if what yo want is not available Aggressive: steal a resorce if what yo want is not available Hierarchical allocation: Assign resorces to classes Can only ask for resorces from a higher nmber class than what yo hold now CS 153 Lectre 13 Deadlock 14

13 Deadlock Avoidance Prevention can be too conservative can we do better? Avoidance Togh Provide information in advance abot what resorces will be needed by processes System only grants resorce reqests if it knows that deadlock cannot happen Avoids circlar dependencies Hard to determine all resorces needed in advance Good theoretical problem, not as practical to se CS 153 Lectre 13 Deadlock 15

14 Banker s Algorithm The Banker s Algorithm is the classic approach to deadlock avoidance for resorces with mltiple nits 1. Assign a credit limit to each cstomer (process)» Maximm credit claim mst be stated in advance 2. Reject any reqest that leads to a dangeros state» A dangeros state is one where a sdden reqest by any cstomer for the fll credit limit cold lead to deadlock» A recrsive redction procedre recognizes dangeros states 3. In practice, the system mst keep resorce sage well below capacity to maintain a resorce srpls» Rarely sed in practice de to low resorce tilization CS 153 Lectre 13 Deadlock 16

15 Possible System States Unsafe Deadlock Safe CS 153 Lectre 13 Deadlock 17

16 Banker s Algorithm Simplified OK OK OK UNSAFE P 1 P 2 P 1 P 2 P 1 P 2 P 1 P 2 CS 153 Lectre 13 Deadlock 18

17 Detection and Recovery Detection and recovery If we don t have deadlock prevention or avoidance, then deadlock may occr In this case, we need to detect deadlock and recover from it To do this, we need two algorithms One to determine whether a deadlock has occrred Another to recover from the deadlock Possible, bt expensive (time consming) Implemented in VMS Rn detection algorithm when resorce reqest times ot CS 153 Lectre 13 Deadlock 19

18 Deadlock Detection Detection Traverse the resorce graph looking for cycles If a cycle is fond, preempt resorce (force a process to release) Expensive Many processes and resorces to traverse Only invoke detection algorithm depending on How often or likely deadlock is How many processes are likely to be affected when it occrs CS 153 Lectre 13 Deadlock 20

19 Deadlock Recovery Once a deadlock is detected, we have two options 1. Abort processes Abort all deadlocked processes» Processes need to start over again Abort one process at a time ntil cycle is eliminated» System needs to rern detection after each abort 2. Preempt resorces (force their release) Need to select process and resorce to preempt Need to rollback process to previos state Need to prevent starvation CS 153 Lectre 13 Deadlock 21

20 Deadlock Smmary Deadlock occrs when processes are waiting on each other and cannot make progress Cycles in Resorce Allocation Graph (RAG) Deadlock reqires for conditions Mtal exclsion, hold and wait, no resorce preemption, circlar wait For approaches to dealing with deadlock: Ignore it Living life on the edge Prevention Make one of the for conditions impossible Avoidance Banker s Algorithm (control allocation) Detection and Recovery Look for a cycle, preempt or abort CS 153 Lectre 13 Deadlock 22

21 Other Problems with Sync. Performance problems Lock contention» Only for spinlocks?» No; consider blocking locks Lock convoying: several processes need locks in the same order. Slow process gets in first CS 153 Lectre 13 Deadlock 23

22 Advanced Synchronization Lock free data strctres Can we avoid sing locks? Transactional memory (e.g., Intel TSX) System spport for lock free operation CS 153 Lectre 13 Deadlock 24

23 Lock-Free Data Strctres Assme compare and swap atomic instrction Limitation: swap a single memory location Only spported on some processor architectres Rewrite critical section Create copy of data strctre Modify copy Swap in pointer to copy iff no one else has Restart if pointer has changed CS 153 Lectre 13 Deadlock 25

24 Smmary Schedling vs. Synchronization Passive vs. active Problems: starvation, deadlock» Q: can schedling has deadlock? Synchronization mechanisms Lock: basic Semaphore: more abstracted Monitor and C/V: even more abstracted Q: are higher level mechanisms always better? CS 153 Lectre 13 Deadlock 27

25 Next class Midterm review CS 153 Lectre 13 Deadlock 28