What does the "lock" instruction mean in x86 assembly?

 

Code

#include <atomic>

#include <cassert>

#include <iostream>

#include <thread>

#include <vector>

std::atomic_ulong my_atomic_ulong(0);

unsigned long my_non_atomic_ulong = 0;

unsigned long my_arch_atomic_ulong = 0;

unsigned long my_arch_non_atomic_ulong = 0;

size_t niters;

void threadMain() {

for (size_t i = 0; i < niters; ++i) {

my_atomic_ulong++;

my_non_atomic_ulong++;

__asm__ __volatile__ (

"incq %0;"

: "+m" (my_arch_non_atomic_ulong)

:

:

);

__asm__ __volatile__ (

"lock;"

"incq %0;"

: "+m" (my_arch_atomic_ulong)

:

:

);

}

}

int main(int argc, char **argv) {

size_t nthreads;

if (argc > 1) {

nthreads = std::stoull(argv[1], NULL, 0);

} else {

nthreads = 2;

}

if (argc > 2) {

niters = std::stoull(argv[2], NULL, 0);

} else {

niters = 10000;

}

std::vector<std::thread> threads(nthreads);

for (size_t i = 0; i < nthreads; ++i)

threads[i] = std::thread(threadMain);

for (size_t i = 0; i < nthreads; ++i)

threads[i].join();

assert(my_atomic_ulong.load() == nthreads * niters);

assert(my_atomic_ulong == my_atomic_ulong.load());

std::cout << "my_atomic_ulong " << my_atomic_ulong << std::endl;

std::cout << "my_non_atomic_ulong " << my_non_atomic_ulong << std::endl;

assert(my_arch_atomic_ulong == nthreads * niters);

std::cout << "my_arch_atomic_ulong " << my_arch_atomic_ulong << std::endl;

std::cout << "my_arch_non_atomic_ulong " << my_arch_non_atomic_ulong << std::endl;

}

Results

g++ -ggdb3 -O0 -std=c++11 -Wall -Wextra -pedantic -o main.out main.cpp -pthread
./main.out 2 10000

ricky@ricky-gn41:~/play/playatomicadd$ ./main.out 2 10000

my_atomic_ulong 20000

my_non_atomic_ulong 19494

my_arch_atomic_ulong 20000

my_arch_non_atomic_ulong 19405

ricky@ricky-gn41:~/play/playatomicadd$ ./main.out 2 10000

my_atomic_ulong 20000

my_non_atomic_ulong 17804

my_arch_atomic_ulong 20000

my_arch_non_atomic_ulong 17977

End