Threads and Critical Sections
Vivek Pai / Kai Li
Princeton University
Gedankenthreads
What happens during fork?
We need particular mechanisms, but do
we have options about what to do?
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Mechanics
Midterm grading finished!
Tabulating, recording not done
Available this afternoon, really!
Let’s talk a little about the midterm
Start threads & critical sections
Readings will be updated & mentioned
in followup (so you’re responsible  )
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Thread and Address Space
Thread
A sequential execution stream within a
process (also called lightweight process)
Address space
All the state needed to run a program
Provide illusion that program is running on
its own machine (protection)
There can be more than one thread per
address space
4
Concurrency and Threads
I/O devices
Overlap I/Os with I/Os and computation (modern
OS approach)
Human users
Doing multiple things to the machine: Web
browser
Distributed systems
Client/server computing: NFS file server
Multiprocessors
Multiple CPUs sharing the same memory: parallel
program
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Typical Thread API
Creation
Fork, Join
Mutual exclusion
Acquire (lock), Release (unlock)
Condition variables
Wait, Signal, Broadcast
Alert
Alert, AlertWait, TestAlert
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User vs. Kernel-Level Threads
Question
What is the difference between user-level and
kernel-level threads?
Discussions
When a user-level thread is blocked on an I/O
event, the whole process is blocked
A context switch of kernel-threads is expensive
A smart scheduler (two-level) can avoid both
drawbacks
7
Thread Control Block
Shared information
Processor info: parent process, time, etc
Memory: segments, page table, and stats, etc
I/O and file: comm ports, directories and file
descriptors, etc
Private state
State (ready, running and blocked)
Registers
Program counter
Execution stack
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Threads Backed By Kernel Threads
Each thread has
a user stack
a private kernel stack
Pros
concurrent accesses
to system services
works on a
multiprocessor
Cons
More memory
Each thread has
a user stack
a shared kernel stack with
other threads in the same
address space
Pros
less memory
Does not work on a
multiprocessor
Cons
serial access to system
services
9
“Too Much Milk” Problem
Person A
Look in fridge: out of milk
Leave for Wawa
Arrive at Wawa
Buy milk
Arrive home
Person B
Look in fridge: out of milk
Leave for Wawa
Arrive at Wawa
Buy milk
Arrive home
Don’t buy too much milk
Any person can be distracted at any point
10
A Possible Solution?
if ( noMilk ) {
if (noNote) {
leave note;
buy milk;
remove note;
}
}
if ( noMilk ) {
if (noNote) {
leave note;
buy milk;
remove note;
}
}
Thread can get context switched after
checking milk and note, but before buying
milk
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Another Possible Solution?
Thread A
Thread B
leave noteA
if (noNoteB) {
if (noMilk) {
buy milk
}
}
remove noteA
leave noteB
if (noNoteA) {
if (noMilk) {
buy milk
}
}
remove noteB
Thread A switched out right after
leaving a note
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Yet Another Possible Solution?
Thread A
leave noteA
while (noteB)
do nothing;
if (noMilk)
buy milk;
remove noteA
Thread B
leave noteB
if (noNoteA) {
if (noMilk) {
buy milk
}
}
remove noteB
Safe to buy
If the other buys, quit
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Remarks
The last solution works, but
Life is too complicated
A’s code is different from B’s
Busy waiting is a waste
Peterson’s solution is also complex
What we want is:
Acquire(lock);
if (noMilk)
buy milk;
Release(lock);
}
Critical section
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What Is A Good Solution
Only one process inside a critical
section
No assumption about CPU speeds
Processes outside of critical section
should not block other processes
No one waits forever
Works for multiprocessors
15
Primitives
We want to avoid thinking (repeatedly)
So, we want some “contract” that
provides certain behavior
Low-level behavior encapsulated in
“primitives”
Application uses primitives to construct
more complex behavior
16
The Simplistic Acquire/Release
Acquire() {
disable interrupts;
}
Release() {
enable interrupts;
}
Kernel cannot let users disable interrupts
Critical sections can be arbitrarily long
Used on uniprocessors, but won’t work on
multiprocessors
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Disabling Interrupts
Done right, serializes activity
People think sequentially – easier to reason
Guarantees code executes without
interruption
Delays handling of external events
Used throughout the kernel
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Using Disabling Interrupts
Acquire(lock) {
disable interrupts;
while (lock != FREE){
enable interrupts;
disable interrupts;
}
lock = BUSY;
enable interrupts;
}
Release(lock) {
disable interrupts;
lock = FREE;
enable interrupts;
}
Why do we need to disable interrupts at all?
Why do we need to enable interrupts inside
the loop in Acquire?
19
Using Disabling Interrupts
Acquire(lock) {
disable interrupts;
while (lock == BUSY) {
enqueue me for lock;
block;
} else
lock = BUSY;
enable interrupts;
}
Release(lock) {
disable interrupts;
if (anyone in queue) {
dequeue a thread;
make it ready;
}
lock = FREE;
enable interrupts;
}
When does Acquire re-enable interrupts in
going to sleep?
Before enqueue?
After enqueue but before block?
20
Hardware Support for Mutex
Mutex = mutual exclusion
Early software-only approaches limited
Hardware support became common
Various approaches:
Disabling interrupts
Atomic memory load and store
Atomic read-modify-write
L. Lamport, “A Fast Mutual Exclusion
Algorithm,” ACM Trans. on Computer
Systems, 5(1):1-11, Feb 1987. – use Google
to find
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The Big Picture
Concurrent Applications
High-Level Locks
Atomic API
Low-Level
Atomic Ops
Semaphores Monitors Send/Receive
Load/Store Interrupt disable Test&Set
Interrupt (timer or I/O completion), Scheduling, Multiprocessor
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Atomic Read-Modify-Write
Instructions
Test&Set: Read value and write 1 back to memory
Exchange (xchg, x86 architecture)
Swap register and memory
Compare and Exchange (cmpxchg, 486+)
If Dest = (al,ax,eax), Dest = SRC;
else (al,ax,eax) = Dest
LOCK prefix in x86
Load link and conditional store (MIPS, Alpha)
Read value in one instruction, do some operations
When store, check if value has been modified. If not, ok;
otherwise, jump back to start
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A Simple Solution with Test&Set
Acquire(lock) {
while (!TAS(lock))
;
}
Release(lock) {
lock = 0;
}
Waste CPU time
Low priority threads may never get a
chance to run
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Test&Set, Minimal Busy Waiting
Acquire(lock) {
while (!TAS(lock.guard))
;
if (lock.value) {
enqueue the thread;
block and lock.guard = 0;
} else {
lock.value = 1;
lock.guard = 0;
}
}
Release(lock) {
while (!TAS(lock.guard))
;
if (anyone in queue) {
dequeue a thread;
make it ready;
} else
lock.value = 0;
lock.guard = 0;
}
Why does this work?
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