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Deadlock Solutions: Avoidance, Detection, and Recovery
CS 241March 30, 2012University of Illinois
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Deadlock: definition
There exists a cycle of processes such that each process cannot proceed until the next process takes some specific action.Result: all processes in the cycle are stuck!
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Deadlock solutions
Prevention• Design system so that deadlock is impossible
Avoidance• Steer around deadlock with smart scheduling
Detection & recovery• Check for deadlock periodically• Recover by killing a deadlocked processes and releasing its
resources
Do nothing• Prevention, avoidance and detection/recovery are expensive• If deadlock is rare, is it worth the overhead?• Manual intervention (kill processes, reboot) if needed
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Last time: Deadlock Prevention
#1: No mutual exclusion• Thank you, Captain Obvious
#2: Allow preemption• OS can revoke resources from current owner
#3: No hold and wait• When waiting for a resource, must not currently hold any
resource
#4: Request resources in order• When waiting for resource i, must not currently hold any
resource j > i• As you can see: If your program satisfies #3 then it
satisfies #4
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“Request In Order” is more permissive
Will not deadlock Might deadlock(depending on
scheduler, inputs, etc.)
Definitelydeadlock
Request in order
No holdand wait
All programs
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Are we always in trouble without ordering resources?
No, not always:
Ordered resource requests are sufficient to avoid deadlock, but not necessaryConvenient, but may be conservative
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Q: What’s the rule of the road?
What’s the law? Does it resemble one of the rules we saw?
I can almost get acrossDrat!
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Deadlock Detection& Recovery
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Deadlock Detection
Check to see if a deadlock has occurred!Special case: Single resource per type
• E.g., mutex locks (value is zero or one)• Check for cycles in the resource allocation graph
General case• E.g., semaphores, memory pages, ...• See book, p. 355 – 358
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Dependencies between processes
Resource allocation
graph
Corresponding process
dependency graph
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Deadlock Recovery
Recovery idea: get rid of the cycles in the process dependency graphOptions:
• Kill all deadlocked processes• Kill one deadlocked process at a
time and release its resources• Steal one resource at a time• Roll back all or one of the
processes to a checkpoint that occurred before they requested any resources, then continue
Difficult to prevent indefinite postponement
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Deadlock Recovery
Have to kill one more
Resource allocation
graph
Corresponding process
dependency graph
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Deadlock Recovery
Only have to kill one
Resource allocation
graph
Corresponding process
dependency graph
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Deadlock Recovery
How should we pick a process to kill?We might consider...
• process priority• current computation time and time to completion• amount of resources used by the process• amount of resources needed by the process to complete• the minimal set of processes we need to eliminate to
break deadlock• is process interactive or batch?
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Rollback instead of killing processesSelecting a victim
• Minimize cost of rollback (e.g., size of process’s memory)
Rollback• Return to some safe state• Restart process for that state• Note: Large, long computations are sometimes
checkpointed for other reasons (reliability) anyway
Challenge: Starvation• Same process may always be picked as victim• Fix: Include number of rollbacks in cost factor
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Deadlock Avoidance
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Deadlock Avoidance
Idea: Steer around deadlock with smart schedulingAssume OS knows:
• Number of available instances of each resource Each individual mutex lock is a resource with one instance
available Each individual semaphore is a resource with possibly
multiple “instances” available• For each process, current amount of each resource it owns• For each process, maximum amount of each resource it might
ever need For a mutex this means: Will the process ever lock the mutex?
Assume processes are independent• If one blocks, others can finish if they have enough resources
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How to guide the system down a safe path of executionHelper function: is a given state safe?
• Safe = there’s definitely a way to finish the processes without deadlock
When a resource allocation request arrives• Pretend that we approve the request• Call function: Would we then be safe? • If safe,
Approve request• Otherwise,
Block process until its request can be safely approved Some other process is scheduled in the meantime
This is called the Banker’s Algorithm• Dijkstra, 1965
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What is a state?
For each resource,• Current amount available• Current amount allocated to each process• Future amount needed by each process (maximum)
Free
P1 alloc
P2 alloc
P1 need
P2 need
Buffer space A mutex
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When is a state safe?
There is an execution order that can finishIn general, that’s hard to predict
• So, we’re conservative: find sufficient conditions for safety• i.e., make some pessimistic assumptions
Pessimistic assumptions:• A process might request its maximum resources at any
time• A process will never release its resources until it’s done
All states
Safe
Unsafe
Deadlocked
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Computing safety
“There is an execution order that can finish”Search for an order P1, P2, P3, ... such that:
• P1 can finish using what it has plus what’s free• P2 can finish using what it has + what’s free + what P1
releases when it finishes• P3 can finish using what it has + what’s free + what P1
and P2 will release when they finish• ...
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Computing safety
“There is an execution order that can finish”More specifically... Search for an order P1, P2, P3, ... such that:
• P1’s max resource needs ≤ what it has + what’s free• P2’s max resource needs ≤ what it has + what’s free +
what P1 will release when it finishes• P3’s max resource needs ≤ what it has + what’s free +
what P1 and P2 will release when they finish• ...
But how do we find that order?
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Inspiration
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Playing Pickup Sticks with ProcessesPick up a stick on top
• = Find a process that can finish with what it has plus what’s free
Remove stick• = Process finshes & releases its resources
Repeat until...• ...all processes have finished
Answer: safe• ...or we get stuck
Answer: unsafe
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Try it: is this state safe?
P2 alloc
Buffer space A mutexFree
P2 need
P1 alloc
P1 need
Which process can
go first?
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 needStart with P2
Buffer space A mutex
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 needRelease P2’s
resources
Buffer space A mutex
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 needRelease P2’s
resources
Buffer space A mutex
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 needContinue with
P1
Buffer space A mutex
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 needContinue with
P1
Buffer space A mutex
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Try it: is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 need
Yes, it’s safe: Order is P2,
P1
Buffer space A mutex
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Example 2: Is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 need
P3 alloc
P3 need
Can P1 go first?
Can P2 go first?
Can P3 go first?
Buffer space
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Example 2: Is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 need
P3 alloc
P3 need
Buffer space
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Example 2: Is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 need
P3 alloc
P3 need
Buffer space
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Example 2: Is this state safe?
P2 alloc
Free
P2 need
P1 alloc
P1 need
P3 alloc
P3 need
Unsafe!
Can P1 go next?
Can P2 go next?
Buffer space
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Deadlock Summary
Deadlock: cycle of processes/threads each waiting for the next
• Nasty timing-dependent bugs!
Detection & Recovery• Typically very expensive to kill / checkpoint processes
Avoidance: steer around deadlock• Requires knowledge of everything an application will request• Expensive to perform on each scheduling event
Prevention (ordered resources)• Imposes conservative rules on application that preclude
deadlock• Application can do it; no special OS support
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Deadlock Summary
Typical solution:• OS (Unix/Windows) do nothing (Ostrich Algorithm)• Application uses general-purpose deadlock prevention
Transaction systems (e.g., credit card processing) may use detection/recovery/avoidance