Race condition

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Race condition

• The scourge of parallel and distributed
  computing...

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Race condition

• When multiple processes compete for a
  non-sharable resource
• With no synchronization there is a race to who
  claims/modifies the resource
• Read-Modify-Write (safe)
• Read-Modify-Read-Write-Modify-Write

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Race condition (cont)
mem
Thread A
Thread B
reg mem reg reg 8 mem reg
reg mem reg reg 1 mem reg
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Race condition (cont)
Time reg of A reg of B mem
1 0
2 0 (regmem) 0 (regmem)
3 1 (reg reg1) 8 (reg reg8)
4 ?
5
6
7
reg mem reg reg n mem reg
The race to write to mem creates a wrong result.
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Race condition (cont)
Time reg of A reg of B mem
1 0
2 0 (regmem) 0
3 1 (reg reg1) 0 (regmem)
4 8 (reg reg8) 1 (memA.reg)
5 8 (memB.reg)
6
7
reg mem reg reg n mem reg
The update from process A is lost because it was
not seen by B
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Race condition (cont)
Time reg of A reg of B mem
1 0
2 0 (regmem)
3 1 (reg reg1)
4 1 (memA.reg)
5 1 (regmem)
6 9 (reg reg8)
7 9 (memB.reg)
reg mem reg reg n mem reg
The two updates do not interact, result is as
expected. By sheer luck, operations were
serialized.
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Non-serialized operations
Resource
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Serialized operations
Resource
Change request 1
Change request 2
Change request 3

• The resource is protected by a mechanism which
• enforces serial access to it
• Operating system
• Database manager
• Programming language synchronization

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Synchronization

• Only one process may execute the critical section
  of code
• Acquire exclusive rights
• Execute critical section
• Release exclusive rights

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Synchronization
... acquire_permission(s) reg mem reg reg
n mem reg release_permission(s) ...
The entity s controls access to mem.
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Synchronization

• Semaphore guards n resources
• Any thread can return a resource
• Mutex guards 1 resource
• Only the currently owning thread can return the
  resource
• Threads block until the resource is granted

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Semaphore

• Counts the nof available resources
• Acquire decrement, Release increment
• No allocation or identification of resources
• An atomically modifiable counter
• Race conditions over the semaphore are prevented
  by
• virtual machine / pgm language interpreter
• operating system
• hardware

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Semaphore caveats

• Processes (i.e. programmers) must follow the
  protocol and not
• Forget to return a resource after use
• Return a resource that was not requested
• Hold a resource for too long
• Use a resource anyway

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Mutex

• A binary semaphore (one resource)
• The process acquiring the resource is the only
  one that can return it (ownership)
• The synchronized keyword in Java use any object
  as a monitor (a kind of mutex).

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Monitor

• A mutex with a thread queue
• Threads queue for the mutex
• Threads can yield the resource (wait)
• Threads can alert other threads (notify)
• In Java, every object has a monitor
• The synchronized keyword

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Unsharable resources

• Process memory (part of)
• Files
• Databases
• Printers, scanners, cameras, modems

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Deadlock

• Processes wait forever for all required resources
  to be released

Waiting for R1 to be released
R1
P2
P1
R2
Waiting for R2 to be released
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Livelock

• Processes attempt to break the deadlock by
  time-out and release, but no-one wins

Waiting for R1 to be released
R1
P2
P1
R2
Waiting for R2 to be released
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Further fun reading

• Dijkstra, The Dining Philosophers problem
• Chandy/Misra solution to DPh problem
• Producer-consumer problem
• Sleeping barber problem
• Readers-writers problem