Transcript Spinlocks and Atomic Load/Store

Synchronization
(other solutions …)
Announcements
• Assignment 2 is graded
• Project 1 is due today
Critical Section Problem
• Problem: Design a protocol for processes to cooperate,
such that only one process is in its critical section
– How to make multiple instructions seem like one?
Process 1
CS1
Process 2
CS2
Time 
Processes progress with non-zero speed, no assumption on clock speed
Used extensively in operating systems:
Queues, shared variables, interrupt handlers, etc.
Solution Structure
Shared vars:
Initialization:
Process:
...
...
Entry Section
Critical Section
Exit Section
Added to solve the CS problem
Solution Requirements
• Mutual Exclusion
– Only one process can be in the critical section at any time
• Progress
– Decision on who enters CS cannot be indefinitely postponed
• No deadlock
• Bounded Waiting
– Bound on #times others can enter CS, while I am waiting
• No livelock
• Also efficient (no extra resources), fair, simple, …
Solution 1 (Two Processes)
Shared: int turn;
Initialize: turn = 0;
Process 0
Process 1
turn = 0;
while(turn != 0);
turn = 1;
while(turn != 1);
Critical Section
Critical Section
turn = 1;
turn = 0;
Problem: Does not satisfy progress! Assumes strict alternation
Solution 1 (Two Processes)
Shared: bool flag[2];
Initialize: flag[0] = false; flag[1] = false;
Process 0
Process 1
flag[0] = true;
while(flag[1]);
flag[1] = true;
while(flag[0]);
Critical Section
Critical Section
flag[0] = false;
flag[1] = false;
Problem: Does not satisfy progress! Timing dependent
What if you swap statement in the entry section?
Peterson’s Algorithm
Shared: bool flag[2];
int turn;
Initialize: flag[0] = false;
flag[1] = false;
turn = 0;
Process i
flag[i] = true;
turn = j;
while(flag[j] && turn == j);
Critical Section
flag[i] = false;
Works for atomic load/stores!
Special problem for multiprocessors
Solution 2: Disable Interrupts
• Disable interrupts while process is in critical section
• 2 approaches:
– Scheduler checks if program counter is in critical section
• Do not interrupt if it is
• Pro: Fast Con: needs compiler support
– Process disables interrupts before entering critical section
• Enables on exit
• Pro: Easy Con: when infinite loop in CS
• Cons
– Response time
– Multiprocessors
 Critical section should be small!
Entry Section
Process
Disable interrupts
Critical Section
Exit Section
Enable interrupts
Solution 3: Hardware Primitives
• Modern hardware provides better atomic instructions:
–
–
–
–
–
test_and_set
swap
compare_and_swap
load_linked/store_conditional
fetch_and_add
bool test_and_set(bool *target)
{
bool temp = *target;
*target = TRUE;
return temp;
}
void swap(bool *a, bool *b)
{
bool temp = *a;
*a = *b;
*b = temp;
}
• Entire function is atomic
– Possible by locking bus for both load and store
Solution 3: Hardware Primitives
Share: int lock;
Initialize: lock = false;
Process i
While(test_and_set(&lock));
Critical Section
lock = false;
Problem: Does not satisfy bounded waiting
(see book for correct solution)
Hardware Primitives
• Pros:
– Simple primitive
– Easy to program critical sections
• Cons
– Busy waiting for entire duration of critical section!
• Also called spinlocks
Implementing Semaphores
• P() and V() should be executed atomically
– Disable interrupts, or
– Busy waiting solutions
• Move busy wait from entry of CS to P() and V()
• P() and V() code is small
Semaphores Atomicity
typedef struct semaphore {
int lock;
int value:
ProcessList L;
} Semaphore;
void P(Semaphore *S) {
while(test_and_set(&S->lock) == 1) /* do nothing */;
S->value = S->value - 1;
if (S.value < 0) {
add this process to S.L;
atomic_clear_and block(&S->lock);
} else
atomicclear(&S->lock);
}
void V(S) {
while(test_and_set(&S->lock) == 1) /* do nothing */;
S->value = S->value + 1;
if (S->value <= 0) {
remove a process P from S.L;
wakeup P
}
atomicclear(&S->lock);
}
A Real Race Condition
• Customer to sales executive:
“ This is the second time I have written to you, and I don't
blame you for not answering me, because I sounded
crazy, but it is a fact that as a tradition in our family we
have ice-cream for dessert after dinner each night. …”