CS110 Lecture 14: Threads and Mutexes

CS110: Principles of Computer Systems

Winter 2021-2022

Stanford University

Instructors: Nick Troccoli and Jerry Cain

The Stanford University logo
https://comic.browserling.com/53

CS110 Topic 3: How can we have concurrency within a single process?

Learning About Multithreading

Introduction to Threads

Mutexes and Condition Variables

Semaphores

Multithreading Patterns

Lecture 13

This/next lecture

Lecture 16

Lectures 17/18

assign5: implement your own multithreaded news aggregator to quickly fetch news from the web!

Learning Goals

  • Discover some of the pitfalls of threads sharing the same virtual address space
  • Understand how to identify critical sections and fix race conditions/deadlock
  • Learn how locks can help us limit access to shared resources

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

We can have concurrency within a single process using threads: independent execution sequences within a single process.

  • Threads let us run multiple functions in our program concurrently
  • Multithreading is very common to parallelize tasks, especially on multiple cores
  • In C++: spawn a thread using thread() and the thread variable type and specify what function you want the thread to execute (optionally passing parameters!)
  • Thread manager switches between executing threads like the OS scheduler switches between executing processes
  • Each thread operates within the same process, so they share a virtual address space (!) (globals, text, data, and heap segments)
  • The processes's stack segment is divided into a "ministack" for each thread.
  • Many similarities between threads and processes; in fact, threads are often called lightweight processes.

Multithreading

C++ thread

A thread object can be spawned to run the specified function with the given arguments.

thread myThread(myFunc, arg1, arg2, ...);
  • myFunc: the function the thread should execute asynchronously
  • args: a list of arguments (any length, or none) to pass to the function upon execution
  • Once initialized with this constructor, the thread may execute at any time!
  • Thread function's return value is ignored (can pass by reference instead)

To pass objects by reference to a thread, use the ref() function:

void myFunc(int& x, int& y) {...}

thread myThread(myFunc, ref(arg1), ref(arg2));

C++ thread

For multiple threads, we must wait on a specific thread one at a time:

thread friends[5];

...

for (size_t i = 0; i < 5; i++) {
	friends[i].join();
}

To wait on a thread to finish, use the .join() method:

thread myThread(myFunc, arg1, arg2);

... // do some work

// Wait for thread to finish (blocks)
myThread.join();

Race Conditions

  • Like with processes, threads can execute in unpredictable orderings.
  • A thread-safe function is one that will always execute correctly, even when called concurrently from multiple threads.
  • printf is thread-safe, but operator<< is not.  This means e.g. cout statements could get interleaved!
  • To avoid this, use ​oslock and osunlock (custom CS110 functions - #include "ostreamlock.h") around streams.  They ensure at most one thread has permission to write into a stream at any one time.
cout << oslock << "Hello, world!" << endl << osunlock;
for (size_t i = 0; i < kNumFriends; i++) {
    friends[i] = thread(greeting, ref(i)); 
}  

_start

greeting

main

argc

argv

i

args

args

args

args

args

args

created thread stacks

main stack

Here, we can just pass by copy instead.  But keep an eye out for consequences of shared memory!

Threads Share Memory

  • Threads allow a process to parallelize a problem across multiple cores
  • Consider a scenario where we want to sell 250 tickets and have 10 cores
  • Simulation: let each thread help sell tickets until none are left
int main(int argc, const char *argv[]) {
    thread ticketAgents[kNumTicketAgents];
    size_t remainingTickets = 250;
    
    for (size_t i = 0; i < kNumTicketAgents; i++) {
        ticketAgents[i] = thread(sellTickets, i, ref(remainingTickets));
    }
    for (thread& ticketAgent: ticketAgents) {
        ticketAgent.join();
    }
    
    cout << "Ticket selling done!" << endl;
    return 0;
}

Thread-Level Parallelism

$ ./confused-ticket-agents
....
Thread #1 sold a ticket (7 remain).
Thread #5 sold a ticket (6 remain).
Thread #3 sold a ticket (4 remain).
Thread #4 sold a ticket (4 remain).
Thread #2 sold a ticket (3 remain).
Thread #8 sold a ticket (1 remain).
Thread #9 sold a ticket (0 remain).
Thread #0 sold a ticket (0 remain).
Thread #0 sees no remaining tickets to sell and exits.
Thread #9 sees no remaining tickets to sell and exits.
Thread #8 sees no remaining tickets to sell and exits.
Thread #6 sold a ticket (18446744073709551615 remain).
Thread #7 sold a ticket (18446744073709551613 remain).
Thread #1 sold a ticket (18446744073709551613 remain).
...

Output

There is a race condition here!

  • Problem: threads could interrupt each other in between checking tickets and selling them.




  • If a thread evaluates remainingTickets > 0 to be true and commits to selling a ticket, another thread could come in and sell that same ticket before this thread does.
  • This can happen because remainingImages > 0 test and remainingImages-- aren't atomic.
  • Atomicity: externally, the code has either executed or not; external observers do not see any intermediate states mid-execution.
  • We want a thread to do the entire check-and-sell operation without competition.

Overselling Tickets

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (remainingTickets > 0) {
        sleep_for(500);  // simulate "selling a ticket"
        remainingTickets--;
        ...
    }
  • C++ statements aren't inherently atomic. 
  • We assume that assembly instructions are atomic; but even single C++ statements like remainingTickets-- take multiple assembly instructions.

 

 

Atomicity

  • Even if we altered the code to be something like this, it still wouldn't fix the problem:
static void sellTickets(size_t id, size_t& remainingTickets) {
    while (remainingTickets-- > 0) {
        sleep_for(500);  // simulate "selling a ticket"
        ...
    }
// gets remainingTickets
0x0000000000401a9b <+36>:    mov    -0x20(%rbp),%rax
0x0000000000401a9f <+40>:    mov    (%rax),%eax

// Decrements by 1
0x0000000000401aa1 <+42>:    lea    -0x1(%rax),%edx

// Saves updated value
0x0000000000401aa4 <+45>:    mov    -0x20(%rbp),%rax
0x0000000000401aa8 <+49>:    mov    %edx,(%rax)
  • Each core has its own registers that it has to read from
  • Each thread makes a local copy of the variable before operating on it
  • Problem: What if multiple threads do this simultaneously?  They all think there's only 128 tickets remaining and process #128 at the same time!

Atomicity

// gets remainingImages
0x0000000000401a9b <+36>:    mov    -0x20(%rbp),%rax
0x0000000000401a9f <+40>:    mov    (%rax),%eax

// Decrements by 1
0x0000000000401aa1 <+42>:    lea    -0x1(%rax),%edx

// Saves updated value
0x0000000000401aa4 <+45>:    mov    -0x20(%rbp),%rax
0x0000000000401aa8 <+49>:    mov    %edx,(%rax)

It would be nice if we could put the check-and-sell operation behind a "locked door" and say "only one thread may enter at a time to do this block of code".

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

Critical Sections

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

A critical section is a section of code that should be executed transactionally, without competition from other threads.

This means we want critical sections to be atomic; to other observers, it has either executed or not.

If we can fix this issue here, then sellTickets will be a thread-safe function; it will always execute correctly, even when called concurrently from multiple threads.

Critical Sections

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

We will put a lock here with only one key to unlock it.  We will make it such that the lock must be unlocked to proceed.

🔓🔑

Critical Sections

If a thread gets here and the key is available, the thread takes the key, locks the lock, and runs the code while holding onto the key.

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

🔑

thread #1

🔒

Critical Sections

If another thread gets here and the lock is locked, it must wait its turn.

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

thread #2

🔑

thread #1

🔒

waiting...

Critical Sections

When the executing thread gets here, it unlocks the lock and returns the key.  Now another thread may use it.

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

thread #2

thread #1

now it's my turn!

🔓🔑

Critical Sections

When the executing thread gets here, it unlocks the lock and returns the key.  Now another thread may use it.

static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

thread #2

thread #1

🔑

🔒

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

Mutexes

  • We can create this lock-and-key combo by creating a variable of type mutex.
  • A mutex is technically a type of lock; there are others, but we focus just on mutexes
  • When you create a mutex, it is initially unlocked with the key available
  • You call lock() on the mutex to attempt to lock it and take the key
  • You call unlock() on a mutex if you have ownership of it and wish to unlock it and return the key.  That thread continues normally; one waiting thread (if any) then takes the lock and is scheduled to run.
// Assume multiple threads share this same mutex
mutex myLock;

...

myLock.lock();
// only one thread can be executing here at a time
myLock.unlock()

Mutexes

When a thread calls lock():

  • If the lock is unlocked: the thread takes the lock and continues execution
  • If the lock is locked: the thread blocks and waits until the lock is unlocked
  • If multiple threads are waiting for a lock: they all wait until it's unlocked, one receives lock (not necessarily one waiting longest)
// Assume multiple threads share this same mutex
mutex myLock;

...

myLock.lock();
// only one thread can be executing here at a time
myLock.unlock()

A mutex is a way to add a constraint to your multithreaded program: "only one thread may execute this code at a time." We will learn how to add more "constraints" in the coming lectures.

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section
// Assume multiple threads share this same mutex
mutex myLock;

...

myLock.lock();
// only one thread can be executing here at a time
myLock.unlock()

If you don't pass by reference, every thread will get its own mutex copy (its own lock-and-key); thus every thread will be able to acquire its own lock and run the code!

Demo: adding a mutex to ticket agents

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

  • Example: a thread must acquire a lock before proceeding.  We forget to call unlock somewhere, so one thread keeps the lock forever while others are stuck waiting for the lock forever.
static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #1

🔓🔑

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #1

🔒

🔑

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #1

🔒

🔑

"See ya!"

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #1

🔒

🔑

"See ya!"

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #2

Huh.  Guess I gotta wait for the key.

🔒

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

thread #2

*100 years later*

🔒

🤨

Deadlock

Deadlock is a situation where a thread or threads rely on mutually blocked-on resources that will never become available.

static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            counterLock.unlock();
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        ...
    }
    ...
}

We can fix the issue here by making sure to unlock in all scenarios where a thread no longer needs the lock, including when we exit the loop.

Deadlock is a common issue in multiprocessing.  Make sure to trace each thread's possible paths of execution to ensure they always return shared resources like locks.

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section
static void sellTickets(size_t id, size_t& remainingTickets) {
    while (true) {
    
        if (remainingTickets == 0) break;
        remainingTickets--;
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << remainingTickets << " remain)." << endl << osunlock;
                      
    }
    cout << oslock << "Thread #" << id << " sees no remaining tickets to sell and exits." << endl << osunlock;
}

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section
int main(int argc, const char *argv[]) {
    thread ticketAgents[kNumTicketAgents];
    size_t remainingTickets = 250;
    mutex counterLock;
    
    for (size_t i = 0; i < kNumTicketAgents; i++) {
        ticketAgents[i] = thread(sellTickets, i, ref(remainingTickets), ref(counterLock));
    }
    ...
}

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section
static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            counterLock.unlock();
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << myTicket << " remain)." << endl << osunlock;
    }
    ...
}

Mutex Usage

  1. Identify a critical section; a section that only one thread should execute at a time.
  2. Create a mutex and pass it by reference to all threads executing that critical section
  3. Add a line to lock the mutex at the start of the critical section
  4. Add a line to unlock the mutex at the end of the critical section
static void sellTickets(size_t id, size_t& remainingTickets, mutex& counterLock) {
    while (true) {
        size_t myTicket;

        counterLock.lock();
        if (remainingTickets == 0) {
            counterLock.unlock();
            break;
        } else {
            myTicket = remainingTickets;
            remainingTickets--;
            counterLock.unlock();
        }
        
        sleep_for(500);  // simulate "selling a ticket"
        cout << oslock << "Thread #" << id << " sold a ticket (" << myTicket << " remain)." << endl << osunlock;
    }
    ...
}

Critical sections can impact code performance.  Keep critical sections as small as possible!

Mutex Usage

Other times you need a mutex:

  • When there are multiple threads writing to a variable.
  • When there is a thread writing and one or more threads reading

Why do you not need a mutex when there are no writers (only readers)?

Mutex Analogies

https://www.flickr.com/photos/ofsmallthings/8220574255

A mutex is a variable type that is like a "locked door".

You can lock the door:

- if it's unlocked, you go through the door and lock it

- if it's locked, you ​wait for it to unlock first

 

If you most recently locked the door, you can unlock the door:

- door is now unlocked, another may go in now

Mutex Analogies

A mutex is a variable type that is like a "ball in a bucket".

To proceed, you must take the ball from the bucket and hold onto it.

 

If you find the bucket is empty, you must wait for the ball to be returned.

 

When you are done executing, you return the ball to the bucket.

⚾️

🪣

Plan For Today

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

The Race Condition Checklist

Identify shared data that may be modified concurrently.  What shared data is used across threads, passed by reference or globally?

Document and confirm an ordering of events that causes unexpected behavior.  What assumptions are made in the code that can be broken by certain orderings?

Use concurrency directives to force expected orderings and add constraints.  How can we use mutexes, atomic operations, or other constraints to force the correct ordering(s)?

The Race Condition Checklist

Identify shared data that may be modified concurrently.  What shared data is used across threads, passed by reference or globally?  the ticket count.

Document and confirm an ordering of events that causes unexpected behavior.  What assumptions are made in the code that can be broken by certain orderings?

Use concurrency directives to force expected orderings and add constraints.  How can we use mutexes, atomic operations, or other constraints to force the correct ordering(s)?

The Race Condition Checklist

☑︎ Identify shared data that may be modified concurrently.  What shared data is used across threads, passed by reference or globally?  the ticket count.

Document and confirm an ordering of events that causes unexpected behavior.  What assumptions are made in the code that can be broken by certain orderings?  one thread will check-and-sell a ticket at a time.

Use concurrency directives to force expected orderings and add constraints.  How can we use mutexes, atomic operations, or other constraints to force the correct ordering(s)?

The Race Condition Checklist

☑︎ Identify shared data that may be modified concurrently.  What shared data is used across threads, passed by reference or globally?  the ticket count.

☑︎ Document and confirm an ordering of events that causes unexpected behavior.  What assumptions are made in the code that can be broken by certain orderings?  one thread will check-and-sell a ticket at a time.

Use concurrency directives to force expected orderings and add constraints.  How can we use mutexes, atomic operations, or other constraints to force the correct ordering(s)?  add a mutex that must be acquired before checking-and-selling a ticket.

The Race Condition Checklist

☑︎ Identify shared data that may be modified concurrently.  What shared data is used across threads, passed by reference or globally?  the ticket count.

☑︎ Document and confirm an ordering of events that causes unexpected behavior.  What assumptions are made in the code that can be broken by certain orderings?  one thread will check-and-sell a ticket at a time.

☑︎ Use concurrency directives to force expected orderings and add constraints.  How can we use mutexes, atomic operations, or other constraints to force the correct ordering(s)?  add a mutex that must be acquired before checking-and-selling a ticket.

Recap

  • Recap: C++ Threads and overselling tickets
  • Critical Sections
  • Mutexes
  • Deadlock
  • The Race Condition Checklist

 

 

Next time: adding more constraints with condition variables.