C++ condition_variable wait_for not always working - c++

Question

My application start the logThread thread and then cyclically calls lookForSms wich makes a curl call and wait for logThread to read a certain string, but this string may never come so i used a condition_variable wait_for so that eventually (after 5 minutes) it can continue the test.
The problem is at times it waits a lot longer, sometimes 20 min sometimes forever.
How can I make it reliably work?
I'm using Visual Studio Community 2017, here the code:
void lookForSms(CURL *curl) {
sms_found = false;
int i;
CURLcode res;
cout << "\n" << timestamp() << "\t";
res = curl_easy_perform(curl);
if (res == CURLE_OK) {
unique_lock<mutex> lk(F);
analysis << timestampMil() << "\t";
while (cv.wait_for(lk, TIMEOUT_SMS * 1000ms) != cv_status::timeout && !sms_found);
if (sms_found) {
analysis << timestampMil() << "\tSMS RECEIVED\n";
cout << "\tOK";
}
else {
analysis << timestampMil() << "\tSMS LOST\n";
cout << "\tKO";
}
}
}
void logThread() {
string line;
int n, i, count;
unsigned char buf[BUFFER];
while (!close) {
N.lock();
M.lock();
N.unlock();
n = RS232_PollComport(comport, buf, BUFFER);
if (n > 0 && n < BUFFER) {
buf[n] = '\0';
for (i = 0; i < n; i++) {
if (buf[i] == '\n') {
all << timestampMil() << "\t" << line << "\n";
handleLine(line);
line = string();
}
else if (buf[i] != '\r' && buf[i] >= 32) {
line += buf[i];
}
}
}
M.unlock();
}
}
void handleLine(string line) {
if (searchString(line, RCV_SMS)) {
unique_lock<mutex> lk(F);
sms_found = true;
lk.unlock();
cv.notify_all();
}
}
[EDIT] This should have everything it needs to recreate the problem.
#include <iostream>
#include <thread>
#include <mutex>
#include <condition_variable>
#define TIMEOUT_SMS 300
#define DELAY_SMS 10
mutex L, M, N, F;
condition_variable cv;
void logThread() {
while (!close) {
N.lock();
M.lock();
N.unlock();
handleLine("");
Sleep(500);
M.unlock();
}
}
void lookForSms() {
unique_lock<mutex> lk(F);
sms_found = false;
unlocked = false;
//while (cv.wait_for(lk, TIMEOUT_SMS * 1000ms) != cv_status::timeout && !unlocked);
cv.wait_for(lk, 10 * TIMEOUT_SMS * 1000ms, []() {return unlocked; });
if (sms_found) {
cout << "\tOK";
}
else {
cout << "\tKO";
}
}
}
void handleLine(string line) {
if (false) {// the problem is when cv remains locked
unique_lock<mutex> lk(F);
unlocked = true;
sms_found = true;
lk.unlock();
cv.notify_all();
}
}
void stopThreads() {
N.lock();
M.lock();
//stop log thread
close = true;
N.unlock();
M.unlock();
}
int main(int argc, const char * argv[]) {
thread logger(logThread);
for (i = 0; i < repeat; i++) {
lookForSms();
Sleep(DELAY_SMS * 1000);
}
//aquire mutex and set close to true, then join
stopThreads();
//wait for the thread to end
logger.join();
return 0;
}