Mutual Exclusion

1
Mutual Exclusion Synchronization

• Chapter 5a

2
Concurrency

• Mutual Exclusion and Synchronization
• Principles of Concurrency
• Mutual Exclusion
• Software Solutions
• Hardware Solutions
• Semaphores
• Producer/Consumer
• Message Passing
• Monitors

3
Cooperating Processes
Concurrency

• Operating systems allow for the creation and
  concurrent execution of multiple processes
• concurrency can ease program complexity
• concurrency can increase efficiency
• How can the processes work together?
• Flags
• Files
• Messages
• Shared memory

4
Cooperating Processes
Concurrency

• Concurrent processes (or threads) often need to
  share data (maintained either in shared memory or
  files) and resources
• If there is no controlled access to shared data,
  some processes will obtain an inconsistent view
  of this data
• The actions performed by concurrent processes may
  depend on the order in which their execution is
  interleaved
• With no synchronization, results are typically
  not deterministic nor reproducible.

5
Operating Systems must
Concurrency

• The OS must keep track of active processes.
• The OS must allocate and deallocate resources.
• Processor time
• Memory
• Files
• I/O devices
• The OS must protect the data and physical
  resources.
• The results of a process must be independent of
  the speed of execution relative to the speed of
  other concurrent processes.

6
Examples
Concurrency Coherence
void echo() static char chin, chout chin
getchar() chout chin putchar(chout)
static int a, b void P1() void P2()
a a 1 b 2 b b b 1 a
2 a
7
Process Interaction
Concurrency
Potential Control Problems
Process Interaction
Relationship
Degree of Awareness
8
Resource Competition
Concurrency

• Mutual Exclusion
• Critical resource a single non-sharable
  resource.
• Critical section portion of the program that
  accesses a critical resource.
• Deadlock
• Each process owns a resource that the other is
  waiting for.
• Two processes are waiting for communication from
  the other.
• Starvation
• A process is denied access to a resource, even
  though there is no deadlock situation.

9
The Critical-Section Solution
Mutual Exclusion

• Mutual Exclusion
• At any time, at most one process can be in its
  critical section (CS).
• Progress
• Only processes that are not executing in their CS
  can participate in the decision of who will enter
  next in the CS.
• This selection cannot be postponed indefinitely.
• Bounded Waiting
• After a process has made a request to enter its
  CS, there is a bound on the number of times that
  the other processes are allowed to enter their CS
  - otherwise the process will suffer from
  starvation.
• And, of course, no deadlock!
• No assumptions can be made about speed
• Solutions execute some entry code and some exit
  code surrounding critical section

10
Rules of Mutual Exclusion
Mutual Exclusion

• Mutual exclusion must be enforceable.
• A process that halts in its non-critical section
  must not interfere with other processes.
• No deadlock or starvation.
• If a resource is available, entry must be
  permitted without delay.
• No assumptions are to be made about the relative
  process speeds or number of processors.
• A process can remain inside its critical section
  for only a finite time.

11
Turns
Mutual Exclusion Software
repeat while (turn ! me) do no-op ltcritical
sectiongt turn you other stuff until false
12
Bakery Algorithm
Mutual Exclusion Software
repeat choosingi true ni
max(n0, n1, nn-1) 1) choosingi
false for j 0 to n-1 do while choosingj
do no-op while ((nj, j) lt (ni, i) and
(nj ! 0)) do no-op endfor // do
critical section numberi 0 // do
remainder section until false
Wait while someone else is choosing
When there is a tie, use their id number
Could have overflow if we dont reset all numbers
periodically!
More on Bakery Algorithm later
13
Software Solutions
Mutual Exclusion Software

• Turns
• Bakery algorithm works
• Proof is left as an exercise for the reader -)
• Proof by exhaustive cases
• Bakery algorithm is a pain, complex, difficult to
  follow
• Other software solutions
• Dekkers algorithm
• Petersons algorithm
• Can hardware help?
• An atomic instruction support for exclusion

14
Hardware Solutions
Mutual Exclusion

• Interrupt disabling
• not all interrupts need to be disabled
• might not work in multiprocessor environment
• Special machine instructions
• test and set instruction (TSET)
• exchange instruction (XCHG)
• advantages
• works on any number of processors sharing memory
• simple and supports multiple critical sections
• disadvantages
• busy waiting is employed
• starvation and deadlock are possible

15
Test-and-Set
Mutual Exclusion
bool testset(int i) disable interrupts
if (i 0) i 1 enable
interrupts return false else
enable interrupts return true

• Shared variable i (initialized to 0)
• Only the first Pi who sets i enters CS

Process Pi repeat repeat until !testset(b)
CS b0 Stuff until false
16
Mutual Exclusion w/Test-and-Set
Mutual Exclusion
while(true) waitingi true key true
while(waitingi key) if (key
TestAndSet(lock)) ...context switch...
waitingi false // do critical section
// find the next waiting task (round robin) j
(i 1) n while((j ! i) !waitingj)
j (j 1) n if (j i) lock false
else waitingj false // do remainder
17
Mutual Exclusion w/XCHG
Mutual Exclusion

• Shared variable b is initialized to 0
• Each Pi has a local variable k
• The only Pi that can enter CS is the one who
  finds b0
• This Pi excludes all the other Pj by setting b to
  1

Process Pi repeat k1 repeat xchg(k,b)
until k0 CS b0 RS until false