critical section problem

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Critical Section Problem

CIS 450 Winter 2003

Professor Jinhua Guo

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Thread Control Block

• OS’s representation of thread– Registers, PC, SP, scheduling info, etc

• Several different lists of TCBs in OS– Ready list– Blocked lists

• Waiting for disks• Waiting for input• Waiting for events

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Context Switching

• Switching between 2 threads– Change PC to current instruction of new

thread• might need to run old thread in the future

– Must save exact state of first thread– State saved into the TCB

• What must be saved?– Registers (including PC and SP)– What about stack itself?

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Implementing Threads in User Space

A user-level threads package

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Implementing Threads in the Kernel

A threads package managed by the kernel

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Hybrid Implementations

Multiplexing user-level threads onto kernel- level threads

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Independent vs. Cooperating Threads

• Independent threads– No state shared – separate address space

• Cooperating threads– Same address space– Can affect one another– Must be careful!

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Example of Concurrent Programint balance = 100;void deposit (int amount) {

balance += amount; exit(0);

}

int main () {threadCreate(deposit, 10);threadCreate(deposit, 20);waitForAllDone(); /*make sure all children finish*/printf (“The balance is %d”, balance);

}What are the possible output?

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More Concurrent Programming: Linked lists (head is shared)

insert (head, elem) {

Elem->next := head;

head := elem;

}

void *removed (head) {

void *tmp;

tmp := head;

head := head->next;

return t;

}

Assume one thread calls insert and one calls remove.

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Race Condition

• This kind of bug, which only occurs under certain timing conditions, is called a race condition.

• Output depends on ordering of thread execution

• More concretely:(1) two or more threads access a shared variable with

no synchronization, and

(2) at least one of the threads writes to the variable

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Critical Section

• Section of code that:– Must be executed by one thread at a time– If more than one thread executes at a time,

have a race condition– Ex: linked list from before

• Insert/Delete code forms a critical section• What about just the Insert or Delete code?

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Critical Section (CS) Problem

• Provide entry and exit routines– All threads must call entry before executing

CS.– All threads must call exit after executing CS– Thread must not leave entry rounte until it’s

safe.

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Structure of threads for Critical Section Problem

• Threads do the following

While (1) {

call entry();

critical section

call exit();

do other stuff;

}

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Properties of Critical Section Solution

• Mutual Exclusion: at most one thread is executing CS.

• Absence of Deadlock: two or more threads trying to get into CS => at least one succeeds.

• Absence of Unnecessary Delay: if only one thread trying to get into CS, it succeeds

• Bounded Waiting: thread eventually gets into CS.

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Critical Section Solution Attempt #1(2 thread solution)

Intially, turn == 0

entry (id) {while (turn !=id); /* if not my turn, spin */

}

exit(id) {turn = 1 - id; /* other thread’s turn*/

}

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Critical Section Solution Attempt #2(2 thread solution)

Intially, flag[0] == flags[1] == false

entry (id) {flag [id] = true; /* I want to go in */while (flag[1-id]); /* proceed if other not trying */

}

exit(id) {flag[id] = false; /* I am out*/

}

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Critical Section Solution Attempt #3(2 thread solution)

Intially, flag[0] == flags[1] == false, turn

entry (id) {flag [id] = true; /* I want to go in */turn = 1 – id;while (flag[1-id] && turn == 1-id); /* proceed if

other not trying */}exit(id) {

flag[id] = false; /* I am out*/}

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Satisfying the 4 properties

• Mutual exclusion– turn must be 0 or 1 => only one thread can be in CS

• Absence of deadlock– turn must be 0 or 1 => one thread will be allowed in

• Absence of unnecessary delay– only one thread trying to get into CS => flag[other] is

false => will get in

• Bounded Waiting– spinning thread will not modify turn– thread trying to go back in will set turn equal to

spinning thread

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Critical Section Problemmultiple threads solutions

• Bakery algorithm

• It’s a mess– Trying to prove it correct is a headache

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Hardware Support

• Provide instruction that is:– Atomic– Fairly easy for hardware designer to

implement

• Read/Modify/Write– Atomically read value from memory, modify it

in some way, write it back to memory

• Use to develop simpler critical section solution for any number of threads.

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Atomic Operation

• An operation that, once started, runs to completion

• Indivisible

• Ex: loads and stores– Meaning: if thread A stores “1” into variable x

and thread B stores “2” into variable x about the same time, result is either “1” or “2”.

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Test-and-Set

• Many machines have itbool TestAndSet(bool &target) {

bool b = target;

target = true;

return b;

}

• Executes atomically

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CS solution with Test-and-Set

Initially, s == false;

entry () {bool spin;spin = TestAndSet(s);while (spin)

spin = TS(s);}

exit() {S = false;

}

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Basic Idea With Atomic Instructions

• Each thread has a local flag

• One variable shared by all threads

• Use the atomic instruction with flag, shared variable– on a change, all thread to go in– Other threads will not see this change

• When done with CS, set shared varible back to initial state.

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Other Atomic Instructions

The definition of the Swap instruction

void Swap(bool &a, bool &b) {

bool temp = a;

a = b;

b = temp;

}

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Problems with busy-waiting CS solution

• Complicated

• Inefficient– Consumes CPU cycles while spinning

• Priority inversion problem– Low priority thread in CS, high priority thread

spinning can end up causing deadlock

Solution: block when waiting for CS