1. Race Conditions David Ferry CSCI 3500 – Operating Systems
Saint Louis University St. Louis, MO 63103

2. CSCI 3500 - Operating Systems
Definition
A race condition occurs whenever the output of a computation changes depending on the timing of execution. Suppose x=0 initially: Thread 1 Thread 2 u = x v = x u = u + 1 v = v * 2 x = u x = v What are the possible outcome values for x?
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3. Linked List push() Example
Data
Next
NULL
HEAD
push( node* newNode ){ node* current = HEAD; while( current->next != NULL ){ current = current->next; } current->next = newNode; newNode->next = NULL;
CSCI Operating Systems

4. CSCI 3500 - Operating Systems
push() Race
Suppose two threads execute push() simultaneously:
Data
Next
NULL
HEAD
Thread 1:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
Thread 2:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
CSCI Operating Systems

5. CSCI 3500 - Operating Systems
push() Race
Suppose two threads execute push() simultaneously:
Data
Next
NULL
HEAD
current
Thread 1:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
Thread 2:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
CSCI Operating Systems

6. CSCI 3500 - Operating Systems
push() Race
Suppose two threads execute push() simultaneously:
Data
Next
NULL
HEAD
current
current
Thread 1:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
Thread 2:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
CSCI Operating Systems

7. CSCI 3500 - Operating Systems
push() Race
Suppose two threads execute push() simultaneously:
Data
Next
Data
Next
Data
Next
Data
HEAD
Next
NULL
current
current
Thread 1:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
Thread 2:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
CSCI Operating Systems

8. CSCI 3500 - Operating Systems
push() Race
Suppose two threads execute push() simultaneously:
Data
Next
Data
Next
Data
Next
Data
Data
HEAD
Next
NULL
Next
NULL
current
current
Thread 1:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
Thread 2:
push( node* newNode ){
node* current = HEAD;
while( current->next != NULL ){
current = current->next; }
current->next = newNode;
newNode->next = NULL;
CSCI Operating Systems

9. CSCI 3500 - Operating Systems
At least basic arithmetic is safe, right? What could go wrong? Thread 1: Thread 2: x++ x++
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10. Not even increment is safe…
Suppose x=0 initially: Thread 1: Thread 2: x++ x++ Becomes: load x to register load x to register increment register increment register store reg. to memory store reg. to memory
CSCI Operating Systems