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Fix ordering of pthread_create(pthread_t *thread)

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This change fixes a bug where signal_latency_async_test would flake less
than 1/1000 of the time. What was happening was pthread_kill(sender_thr)
would return EFAULT. This was because pthread_create() was not returning
the thread object pointer until after clone() had been called. So it was
actually possible for the main thread to stall after calling clone() and
during that time the receiver would launch and receive a signal from the
sender thread, and then fail when it tried to send a pong. I thought I'd
use a barrier at first, in the test, to synchronize thread creation, but
I firmly believe that pthread_create() was to blame and now that's fixed
This commit is contained in:
Justine Tunney 2025-01-03 17:28:39 -08:00
parent ed6d133a27
commit e939659b70
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GPG key ID: BE714B4575D6E328
6 changed files with 57 additions and 63 deletions
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42
test/posix/signal_latency_async_test.c
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@ -40,11 +40,10 @@ void receiver_signal_handler(int signo) {
}
void *sender_func(void *arg) {
for (int i = 0; i < ITERATIONS; i++) {
// Wait a bit sometimes
if (rand() % 2 == 1) {
if (rand() % 2) {
volatile unsigned v = 0;
for (;;)
if (++v == 4000)
@ -67,32 +66,25 @@ void *sender_func(void *arg) {
}
void *receiver_func(void *arg) {
// Wait for asynchronous signals
for (;;) {
static int iteration = 0;
do {
// wait for signal handler to be called
if (atomic_exchange_explicit(&receiver_got_signal, 0,
memory_order_acq_rel)) {
// record received time
struct timespec receive_time;
clock_gettime(CLOCK_MONOTONIC, &receive_time);
long sec_diff = receive_time.tv_sec - send_time.tv_sec;
long nsec_diff = receive_time.tv_nsec - send_time.tv_nsec;
double latency_ns = sec_diff * 1e9 + nsec_diff;
latencies[iteration++] = latency_ns;
static int iteration = 0;
if (iteration < ITERATIONS)
latencies[iteration++] = latency_ns;
// Pong sender
// pong sender
if (pthread_kill(sender_thread, SIGUSR2))
exit(2);
// Exit if done
if (iteration >= ITERATIONS)
pthread_exit(0);
}
}
} while (iteration < ITERATIONS);
return 0;
}
@ -108,11 +100,7 @@ int compare(const void *a, const void *b) {
int main() {
// TODO(jart): fix flakes
if (1)
return 0;
// Install signal handlers
// install handlers
struct sigaction sa;
sa.sa_handler = receiver_signal_handler;
sa.sa_flags = 0;
@ -121,27 +109,27 @@ int main() {
sa.sa_handler = sender_signal_handler;
sigaction(SIGUSR2, &sa, 0);
// Create receiver thread first
// create receiver thread first
if (pthread_create(&receiver_thread, 0, receiver_func, 0))
exit(11);
// Create sender thread
// create sender thread
if (pthread_create(&sender_thread, 0, sender_func, 0))
exit(12);
// Wait for threads to finish
// wait for threads to finish
if (pthread_join(sender_thread, 0))
exit(13);
if (pthread_join(receiver_thread, 0))
exit(14);
// Compute mean latency
// compute mean latency
double total_latency = 0;
for (int i = 0; i < ITERATIONS; i++)
total_latency += latencies[i];
double mean_latency = total_latency / ITERATIONS;
// Sort latencies to compute percentiles
// sort latencies to compute percentiles
qsort(latencies, ITERATIONS, sizeof(double), compare);
double p50 = latencies[(int)(0.50 * ITERATIONS)];