1. Course Syllabus 1. Introduction - History; Views; Concepts; Structure
2. Process Management - Processes; State + Resources; Threads; Unix implementation of Processes
3. Scheduling – Paradigms; Unix; Modeling
4. Synchronization - Synchronization primitives and their equivalence; Deadlocks
5. Memory Management - Virtual memory; Page replacement algorithms; Segmentation
6. File Systems - Implementation; Directory and space management; Unix file system; Distributed file systems (NFS)
7. Virtualization – Virtual machines, type I and II hypervisors, classic virtualization, sensitive and privileged instructions, binary translation, memory virtualization
7. Distributed Synchronization (if there's time)
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

2. Mutual exclusion: motivation
Race Conditions:
Example: Spooler directory with slots; index variables; two processes attempt concurrent access. In points to the next empty slot, Out points to entry holding name of file to print next.
Process A 4 5 6 7
. . .
. . .
abc.doc
f.ps
paper.ps
In=7
Process B
Out = 4
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

3. Code using reads/writes only
Add-file-to-print-spool (char *name)
Local-in := In
StoreName(Spool-dir[local-in], name)
In := (local-in+1) mod DIR-LEN
A scenario proving the above code wrong
Process A performs line 1
Process A is preempted by Process B
Process B performs Add-file-to-print-spool to completion
Process A is re-scheduled, completes lines 2-3.
Process B's file is never printed.
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

4. The mutual exclusion problem (Dijkstra, 1965)
How can we avoid such race conditions?
We need to devise a protocol that guarantees
mutually exclusive access by processes to a
shared resource (such as a file, printer, etc.) or,
more generally, a critical section of code
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

5. The problem model Multiple processes
Processes can apply read, write, or stronger operations to shared memory
Completely asynchronous
We assume processes do not fail-stop
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

6. Mutual exclusion: formal definition
loop forever
Remainder code
Entry code
Critical section (CS)
Exit code
end loop
Remainder code
Entry code CS
Exit code
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

7. Mutex Requirements
Mutual exclusion: No two processes are at the critical section (CS) at the same time
Deadlock-freedom: If processes are trying to enter their critical section, then some process eventually enters the critical section
Starvation-freedom: If a process is trying to enter its critical section, then this process must eventually enter its critical section
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

8. Mutual exclusion using critical regions
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

9. Brute force solution: disabling interrupts
Disable interrupts
Do your stuff
Enable interrupts
Problems
User processes are not allowed to disable interrupts
Disabling interrupts must be done for a very short period of time
Does not solve the problem in a multi-processor system
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

10. 2-process Mutual Exclusion with a lock variable
initially: lock=0
await c: repeatedly evaluate condition c until true
Program for both processes
await lock=0
lock:=1 CS
lock:=0 No
Does the algorithm satisfy mutex?
Does it satisfy deadlock-freedom?
Does it satisfy starvation-freedom?
Yes No
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

11. Mutual Exclusion: strict alternation
initially: turn=0
Program for process 0
await turn=0 CS
turn:=1
Program for process 1
await turn=1 CS
turn:=0
Yes
Does the algorithm satisfy mutex?
Does it satisfy deadlock-freedom?
Does it satisfy starvation-freedom? No No
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

12. Mutual Exclusion: flag array
bool flags[2] initially {false, false}
Program for process 0
flags[0]:=true
await flags[1]=false CS
flags[0]:=false
Program for process 1
flags[1]:=true
await flags[0]=false CS
flags[1]:=false
Yes
Does the algorithm satisfy mutex?
Does it satisfy deadlock-freedom?
Does it satisfy starvation-freedom? No No
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

13. Peterson’s 2-process algorithm (Peterson, 1981)
initially: b[0]:=false, b[1]:=false, initial value of turn immaterial
Program for process 0
b[0]:=true
turn:=0
await (b[1]=false or turn=1) CS
b[0]:=false
Program for process 1
b[1]:=true
turn:=1
await (b[0]=false or turn=0) CS
b[1]:=false
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

14. Schema for Peterson’s 2-process algorithm
Indicate participation b[i]:=true
Barrier turn:=i
no, maybe
Is there contention? b[1-i]=true?
yes
First to cross barrier? turn=1-i? no
yes
Critical Section
Exit code b[i]:=false
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

15. Peterson’s 2-process algorithm satisfies both mutual-exclusion and starvation-freedom
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

16. Mutual exclusion for n processes: A tournament tree
Level 2 1 2 3 4 5 6 7
Level 1
Level 0
Processes
A tree-node is identified by: [level, node#]
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

17. N-process mutual exclusion: a tournament tree
Variables Per node: b[level, 2node], b[level, 2node+1], turn[level,node]
Per process (local): level, node, id.
Program for process i
node:=i
For level = 0 to log n-1 do
id:=node mod 2
node:= node/2
b[level,2node+id]:=true
turn[level,node]:=id
await (b[level,2node+1-id]=false or turn[level,node]=1-id)
end do CS
for level=log n –1 downto 0 do
node:=  i/2level
b[level,2node+id]:=false // id is value set at this node in entry
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

18. Fairness: First in First Out (FIFO)
Mutual Exclusion
Deadlock-freedom
Starvation-freedom
remainder
doorway
waiting
entry code
critical
section
exit code
FIFO: if process p is waiting and process q has not yet started the doorway, then q will not enter the CS before p
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

19. Lamport’s Bakery Algorithm1 2 3 4 5 n
remainder 1 3 2 2 4
doorway
entry
waiting 1 3 2 4 2 1 2 2 CS
time 1 2 2
exit
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

20. Implementation 1 code of process i , i  {1 ,..., n}
number[i] := 1 + max {number[j] | (1  j  n)}
for j := 1 to n (<> i) {
await (number[j] = 0)  (number[j] > number[i]) }
critical section
number[i] := 0 1 2 3 4 n
number
integer
Does this implementation work?
Answer: No, it can deadlock!
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

21. Implementation 1: deadlock2 3 4 5 n
remainder 1 2 2
doorway
entry
waiting 1 2 2 1
deadlock CS
time 1
exit
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

22. Implementation 2 code of process i , i  {1 ,..., n}
number[i] := 1 + max {number[j] | (1  j  n)}
for j := 1 to n (<> i) {
await (number[j] = 0)  (number[j],j) < number[i],i)
// lexicographical order }
critical section
number[i] := 0 1 2 3 4 n
number
integer
Does this implementation work?
Answer: It does not satisfy mutual exclusion!
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

23. Implementation 2: no mutual exclusion1 2 3 4 5 n
remainder 1 2 2
doorway
entry
waiting 1 2 2 1 2 2 CS
time 1
exit
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili
Synchronization Algorithms and Concurrent Programming , Gadi Taubenfeld © 2006

24. The Bakery Algorithm code of process i, i  {1 ,..., n}
1: choosing[i] := true
2: number[i] := 1 + max {number[j] | (1  j  n)}
3: choosing[i] := false
4: for j := 1 to n do
5: await choosing[j] = false
6: await (number[j] = 0)  (number[j],j)  (number[i],i)
7: od
8: critical section
9: number[i] := 0
Doorway
Waiting
Bakery 1 2 3 4 n
choosing
false
false
false
false
false
false
bits
number
integer
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

25. Lamport’s bakery algorithm satisfies mutual-exclusion, starvation-freedom, and FIFO
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

26. The test-and-set instruction
initially: v:=0
Test-and-set(w)
do atomically
prev:=w
w:=1
return prev
Program for process i
await test&set(v) = 0
Critical Section
v:=0
Mutual exclusion?
Yes
Deadlock-freedom?
Yes
Starvation-freedom? No
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili

27. Starvation-free mutex using test-and-set
boolean lock initially 0, interested[n] initially false
program for process i
interested[i]:=true
await ( (test-and-set(lock) = 0) || (interested[i]=false) ) CS
interested[i]:=false
j:=(i+1) % n
while (j != i && !interested[j]) j:=++j % n
if (j=i)
lock:=0
else
interested[j]:=false
Operating Systems, Spring 2018, I. Dinur, D. Hendler and R. Iakobashvili