Chapter Seven -- Deadlocks -- Lecture Notes

• 7.0 Introduction and Chapter Objectives
  
  
  • Deadlock occurs in a group of processes when each process in the group is waiting for an event that can only be caused by another process in the group.
    
    
  • Computing systems nowadays have increasing numbers of resources, and increasing numbers of processes, which are often long-lived and/or multithreaded. These conditions may lead to more frequent occurrence of deadlock.
    
    
  • Chapter Goals
    • Describe Deadlock
    • Present Methods for Preventing or Avoiding Deadlock
    • Present Methods for Detecting and Recovering from Deadlock
  
  
• 7.1 System Model
  
  
  • Our model system contains processes and resources.
    
    
  • The set of all resources is partitioned into equivalence classes called resource types.
    
    
  • Examples of resources: units of CPU time, printers, drives, units of memory, files, and semaphores.
    
    
  • A running process may request resources at any time.
    
    
  • A process uses resources according to this pattern: request-use-release.
    
    
  • A process may request multiple resources at the same time. However the request may not exceed the number existing in the system. For example if there are two tape drives and four DVD's in the system, the process may request up to (but no more than) two tape drives and four DVD's with one request.
    
    
  • A process must wait until everything it has requested can be granted - all at once, otherwise deadlock can occur (How?)
    
    
  • The request and release operations may be implemented as system calls or through the use of synchronizations tools such as semaphores.
    
    
  • Deadlock occurs in a group of processes when each process in the group is waiting for an event that can only be caused by another process in the group.
    
    
  • A deadlock is stable. None of the events for which the processes are waiting will ever happen.
  
  
• 7.2 Deadlock Characterization
  
  
  • As a simple example of how a deadlock can happen, imagine two processes P₀ and P₁, sharing two semaphores, S₀ and S₁, both initialized with value equal to 1. Assume that P₀ and P₁ are the only processes that have access to S₀ and S₁. Supposed the following things happen in the order given:
    • P₀ waits on S₀
    • P₁ waits on S₁
    • P₀ waits on S₁
    • P₁ waits on S₀
    Now both processes are blocked forever - deadlocked.
    
    
  • Section 7.2.1 -- Necessary Conditions -- the important thing is that deadlock can be prevented by eliminating any one (or more) of these necessary conditions .
    
    
    • Mutual Exclusion -- If multiple processes are permitted to access all resources concurrently then deadlock can't happen.
      
      
    • Hold and Wait -- If processes never hold a resource while waiting for a resource then deadlock can't happen.
      
      
    • No preemption -- If "enough" preemption of resources occurs, then deadlock can't happen. (In the extreme, suppose that whenever a process waits for a resource the system takes away all the resources it currently holds. Deadlock would be impossible. )
      
      
    • Circular Wait -- If cycles in the resource allocation graph cannot occur then there cannot be any deadlock.
      
      
  • Section 7.2.2 -- Resource-Allocation Graph (RAG)
    
    
    • Processes are Nodes -- Circles
      
      
    • Resource Types are Nodes -- Squares
      
      
      • Each instance of the resource type is represented by a dot in the square
        
        
    • A solid request-edge points from a process to a resource type
      
      
    • When a request is granted the request edge is instantaneously transformed into an assignment edge extending from one of the instances inside the resource to the process.
      
      
    • The assignment edge disappears when the process releases the resource.
      
      
    • The request-use-release sequence is reflected in the foregoing description of transitions that occur in the resource allocation graph.
      
      
    • The "if no cycles then no deadlock" rule: If there are no cycles in the RAG of a system, then there is no deadlock in the system.
      
      
    • The CONVERSE of the "if no cycles then no deadlock" rule is FALSE, because some systems can get into a state where there IS a cycle in the RAG, but the system is NOT DEADLOCKED. The text has a diagram that depicts such a situation.
      
      
    • However, there is a PARTIAL CONVERSE to the "if no cycles then no deadlock" rule: If there is a cycle in the RAG, AND there is only one instance of every resource type involved in the cycle, then the processes on the cycle are deadlocked.
      
      
    • Whenever some of the resource types have more than one instance, it is possible for a cycle to exist in the RAG when there is no deadlock.
      
      
    • On the other hand, it is possible for deadlock to occur in systems that have more than one instance of some resource types! The text has a diagram that depicts such a deadlock situation.
  
  
• 7.3 Methods for Handling Deadlock
  
  
  • There are three or four possible OS approaches to deadlock:
    
    
    1. Deadlock Prevention: Make rules about how requests and assignments are done so that one or more of the necessary conditions for deadlock is/are missing.
       
       
    2. Deadlock Avoidance: Place restrictions on assignments only when the system is about to enter an "unsafe state" - from which it could immediately "go out of control" and become deadlocked. (The avoidance algorithms we study require extra, advance information concerning the types and numbers of resources each process could request during its lifetime.)
       
       
    3. Deadlock Detection and Correction: Place no restrictions on the system but notice when deadlock has occurred, and recover.
       
       
    4. Ignore the problem: Maybe deadlocks are rare and we can deal with the problem just by rebooting when we notice that some processes appear "frozen." This will not always be adequate - imagine that the computer is flying a plane. Even if the OS ignores the problem, at least programmers can write applications in ways that handle potential deadlocks among the threads of that application.
  
  
• 7.4 Deadlock Prevention (ways of preventing cycles from forming)
  
  
  • Mutual Exclusion: If X is a resource and the OS immediately grants every request for access to X, then X can never be part of a circular wait - it can never be 'involved' in a deadlock. For example, it is OK for any number of processes to share a read-only file. Obviously sometimes exclusive access to resources is required. So we can't prevent deadlock simply by deciding to make all resources sharable all the time.
    
    
  • Hold and Wait:
    • Method: Require each process to request and be allocated all its resources before it begins execution, or
    • Method: allow a process to request resources only when it has none.
    • Disadvantage of these methods: resources may be allocated but unused for long periods of time, and/or time may be wasted releasing resources, and then requesting them again right away.
    • Disadvantage of these methods: If a process waits for more than one resource at a time, there is no guarantee they will all become available at the same time, and therefore starvation can occur. (Remember, in such cases the OS does not have the option of giving the resources to the process one at a time, because that can cause deadlock.)
    
    
  • No Preemption:
    • Method: If a process P requests a resource that is not immediately available, P immediately loses all resources it holds and is required to wait for the new resource, plus all its 'old' resources.
    • Method: Suppose a process P requests some resources that are held by a process Q. If Q is waiting for a resource then P takes what it wants from Q and this is added to the request for which Q is waiting. (If no process waits on a waiting process then there are no cycles.) If Q is not waiting then P waits. [The case P=Q may need special treatment.] (The OS can harmlessly preempt certain things whose 'state' can be quickly saved and restored, like registers and units of memory. However it is much more of a challenge to recover if the OS takes away something like a printer midway through a print job.)
    • Disadvantage of these methods: More time lost waiting for resources, including the possibility of indefinite postponement when waiting for multiple resources.
    
    
  • Circular Wait:
    • Method: Impose a total ordering on resource types and forbid requests that go against the order. (Also a process may not make consecutive requests for instances of the same resource type.) These rules assure that, in chains of waiting processes, the resource numbers are strictly increasing, and so there cannot be any cycles.
    • Disadvantage of this method: It may lead to longer periods of holding some resources, and thus to decreased availability of resources. Also, it allows starvation to occur in cases where there is more than one instance of some resource types.
  
  
• 7.5 Deadlock Avoidance
  
  
  • The idea of deadlock avoidance is for the OS to recognize unsafe states - states from which the system could slip, out of control, into deadlock. To recognize unsafe states, the OS needs to have information about the possible resource needs of each process.
    
    
  • Section 7.5.1 -- Safe State
    
    
    • Basically an unsafe state is one which will turn in to a deadlock if all processes immediately request their remaining possible needs.
      
      
    • The state is safe if it is not unsafe. If the system state is safe, then even if all processes max out their requests, they will nonetheless be able to finish executing and exit in some order P₀, P₁, P₂, ... , P_n. When each process exits, it gives up its resources. Freed resources become available to the next process in the sequence.
      
      
    • When a system practices deadlock avoidance it uses the following criteria to decide whether to grant resources to a requesting process P. The request is granted if:
      
      
      1. P is not asking to exceed its declared maximum possible needs,
      2. the resources are currently available (free), and
      3. granting the request will leave the system in a safe state.
      
      Typically if P tries to exceed its max, the OS will terminate P. If condition #1 is true but #2 or #3 fails, the system makes P wait for the resources. (They will be granted later - when available and 'safe.')
      
      
    • PROBLEMS WITH THAT: The system is burdened with doing a safety check each time there is a resource request. Processes sometimes have to wait for resources that are available. Resource utilization and throughput can be reduced. There is also the possibility of starvation.
      
      
    • Examples of safe and unsafe systems:

      process    max needs  cur alloc  needs left   free      
         P0         10          5           5         3
         P1          4          2           2
         P2          9          2           7
      

      There's one resource type (tape drives) with twelve instances, allocated as shown. The processes have the indicated max possible needs and remaining needs. The system shown above is safe. {P₁, P₀, P₂} is a safe sequence - the processes could finish executing in that order, even if all the processes first request all of their remaining needs. However, suppose that, starting from the situation depicted in the table above, P₂ requests and is given one more tape drive. Then the state changes to this one:

      process    max needs  cur alloc  needs left   free      
         P0         10          5           5         2
         P1          4          2           2
         P2          9          3           6
      

      This system is unsafe. If all the processes request their remaining needs, the system will be deadlocked. It will be possible for the OS to give P₁ its remaining needs, and possible for P₁ to finish and exit. However, after that there will be only 4 tape drives free, and so both P₀ and P₂ will be kept waiting forever.
      
      
  • Section 7.5.2 -- Resource-Allocation Graph
    
    
    • We can create an augmented resource allocation graph (AUGRAG) by adding (dotted) claim edges from processes to resources, representing each request that each process might make.
      
      
    • If there is just one instance of each resource type, then "unsafe" is equivalent to "cycle in the AUGRAG." This is a conceptually simple way to characterize safety. Cycle detection algorithms typically require O(N²) work, where N is the number of processes in the system. This method does not work when there are multiple instances of some resource types, because in that case there can be a cycle in the AUGRAG of a safe system.
      
      
    • Example: See text figures 7.7 and 7.8 (course slides 7.24 and 7.25).
      
      
  • Section 7.5.3 -- Banker's Algorithm
    
    
    • The Banker's Algorithm is a deadlock avoidance scheme that works when there are multiple instances of resource types. It is generally less efficient than the cycle-detection scheme.
      
      
    • Before it does anything else a new process must declare the maximum number of instances of each resource type that it may need.
      
      
    • Various data structures are required. See the GLOSSARY here.
      
      
    • Section 7.5.3.1 -- Safety Algorithm
      
      
      • Read the algorithm to check for safety here.
        
        
    • Section 7.5.3.2 -- Resource Request Algorithm
      
      
      • Read the banker's resource-request algorithm here.
        
        
    • Section 7.5.3.3 -- An Illustrative Example
      
      
      • See the textbook example worked out in complete detail.
  
  
• 7.6 Deadlock Detection
  
  
  • Another alternative: Have the system run a deadlock detection algorithm and have the system run a recovery algorithm after it detects a deadlock.
    
    
  • Section 7.6.1 -- Single Instance of Each Resource Type
    
    
    • In this case there is a deadlock if and only if there is a cycle in the resource allocation graph. Therefore the OS can detect deadlock by maintaining a RAG that represents the system and doing cycle checks from time to time. There is a more compact graphical representation called a wait-for graph that can be used instead. The algorithm will tend to be more efficient if run on this graph.
      
      
  • Section 7.6.2 -- Several Instances of a Resource Type
    
    
    • If there are multiple instances of some resource types then one can detect deadlock with an algorithm similar to the safety algorithm. (We can view the safety algorithm as checking to see if there would be a deadlock if all the processes were to max out their requests.)
      
      
    • See the deadlock detection algorithm.
    
    
  • Section 7.6.3 -- Detection-Algorithm Usage
    
    
    • In the system model we use, the addition of a request edge to a RAG is always the last step in the creation of a deadlock. If we check for deadlock every time a process blocks requesting a resource, then we will detect each deadlock as soon as it happens. Using our textbook's deadlock detection algorithm, this would require a lot of processing - a lot of overhead.
      
      
    • Since deadlocks are usually quite rare, it may suffice to check for deadlock only about as often as deadlock occurs. We also might make use of heuristics, such as low CPU utilization or the presence of processes in the system that have been waiting for resources for an unusually long time.
  
  
• 7.7 Recovery from Deadlock
  
  
  • Section 7.7.1 -- Process Termination
    
    
    • To break a deadlock the OS can just abort all the deadlocked processes, and reclaim all their resources. However, much of their unfinished work will go to waste.
      
      
    • Instead we can select and abort victim processes one at a time, stopping when the deadlock is broken. This brings up the question of what criteria to use to select victims:
      
      
      • Kill the lowest priority process?
      • Kill the youngest process?
      • Kill the process that has most "life" ahead of it?
      • Kill the process with the fewest stateful resources?
      • Kill the process that needs the most additional resources?
      • Kill the smallest possible number of processes?
      
      Also, there is the problem of determining when the deadlock is broken. Will it be necessary to run an expensive deadlock detection algorithm after each process is aborted?
      
      
    • When the OS aborts a process, its data and/or allotted devices may be left in an incorrect state. Consider a process that was in the midst of printing a file or burning a CD.
      
      
  • Section 7.7.2 -- Resource Preemption
    
    
    • Instead of killing deadlocked processes we can take resources from some and give them to others until the deadlock is broken.
    • We have to consider here too the bases for victim selection.
    • If we don't abort a victim process we will probably need to roll it back and restart it at a point in its execution before it acquired the resources that have been preempted. Problem: How? The easiest thing might be to just roll the process back to its beginning - in effect killing and restarting it.
    
    
  • Starvation is a possibility when we abort or rollback processes and restart them. There is no guarantee that they will not become deadlocked again, and be aborted or rolled back again. We may want to build in a mechanism to "spare" former victims.