Commit 1ade0a0c authored by Michal 'vorner' Vaner's avatar Michal 'vorner' Vaner
Browse files

[2202] Use simpler test for mutex

As Jinmei suggests, a double seems to be not atomic enough, so we can
use that. This is mostly his code, slightly modified (and comments
parent a2bcf6e5
......@@ -52,58 +52,30 @@ TEST(MutexTest, destroyLocked) {
// This test tries if a mutex really locks. We could try that with a deadlock,
// but that's not practical (the test would not end).
// In this test, we try to check if a mutex really locks. We could try that with
// a deadlock, but that's not practical (the test would not end).
// So instead, we try to do some operations from multiple threads that are
// likely to break if not locked. Also, they must run for some time so all
// threads are started and the operation must be complicated enough so the
// compiler won't turn it into some kind of single atomic instruction.
// Instead, we try do to some operation on the same data from multiple threads
// that's likely to break if not locked. Also, the test must run for a while
// to have an opportunity to manifest.
// FIXME: Any idea for a simpler but non-atomic operation that keeps an
// invariant?
// So, we'll have an array of numbers. Each thread will try to repeatedly
// find a number large at least as half of the average, take the number,
// distribute the value across the rest of the positions of the array and
// zero the found position. This operation keeps the sum of the array
// the same. But if two threads access it at the same time, it is likely
// one will add something to the position another one have chosen and
// the other one will then zero it, not taking the new value into account.
// That'd lower the total value of the array.
// We run the threads in opposite directions (so we have no chance of them
// keeping the same distance to each other and not meeting). Also, the indexing
// is performed in a circular manner like with a ring buffer.
const unsigned long long length = 1000;
const unsigned long long iterations = 10000;
const unsigned long long value = 2000;
// Currently we try incrementing a double variable. That one is large enough
// and complex enough so it should not be possible for the CPU to do it as an
// atomic operation, at least on common architectures.
const size_t iterations = 100000;
performStrangeOperation(std::vector<long long unsigned>& array, int direction,
Mutex* mutex)
performIncrement(volatile double* canary, volatile bool* ready_me,
volatile bool* ready_other, Mutex* mutex)
unsigned long long position = 0;
// Loosely (busy) wait for the other thread so both will start
// approximately at the same time.
*ready_me = true;
while (!*ready_other) {}
for (size_t i = 0; i < iterations; ++i) {
Mutex::Locker lock(*mutex);
// Find a place with large enough value
while (array[position % length] < value) {
position += direction;
// Take the value
unsigned long long found_value = array[position % length];
// And distribute it to places following the found one, by
// adding 1 to each.
unsigned long long p2 = position;
while (found_value > 0) {
p2 += direction;
if (p2 % length == position % length) {
++array[p2 % length];
// Zero the distributed value
array[position % length] = 0;
*canary += 1;
......@@ -121,22 +93,19 @@ TEST(MutexTest, swarm) {
// This type has a low chance of being atomic itself, further raising
// the chance of problems appearing.
std::vector<long long unsigned> array(length);
for (size_t i = 0; i < length; ++i) {
array[i] = value;
double canary = 0;
Mutex mutex;
// Run two parallel threads, each in one direction
Thread t1(boost::bind(&performStrangeOperation, array, 1, &mutex));
Thread t2(boost::bind(&performStrangeOperation, array, -1, &mutex));
// Run two parallel threads
bool ready1 = false;
bool ready2 = false;
Thread t1(boost::bind(&performIncrement, &canary, &ready1, &ready2,
Thread t2(boost::bind(&performIncrement, &canary, &ready2, &ready1,
// Check the sum didn't change.
long long unsigned sum = 0;
for (size_t i = 0; i < length; ++i) {
sum += array[i];
EXPECT_EQ(length * value, sum) << "Threads are badly synchronized";
// Check it the sum is the expected value.
EXPECT_EQ(iterations * 2, canary) << "Threads are badly synchronized";
// Cancel the alarm and return the original handler
if (sigaction(SIGALRM, &original, NULL)) {
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