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Date: Sun, 3 Jul 2022 00:38:05 +0800
From: William Tang <galaxyking0419@...il.com>
To: musl@...ts.openwall.com
Subject: Spurious wake up in musl
Hi,
According to the about page, the spurious wake up should not be possible:
musl was the first Linux libc to have ..., the first to have condvars
where newly-arrived waiters can't steal wake events from previous
waiters
However, when I use the following code to test spurious wake up:
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#define COUNT_MAX 1000000
static int64_t counter = 0;
static pthread_cond_t cond_var = PTHREAD_COND_INITIALIZER;
static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
static void *thread_func(void *arg)
{
while (true) {
pthread_mutex_lock(&mutex);
if (counter <= 0)
pthread_cond_wait(&cond_var, &mutex);
if (counter <= 0) {
printf("[worker %p] Spurious wakeup occurred! Counter is
%lld!\n", pthread_self(), counter);
exit(0);
}
if (--counter == 0)
printf("[worker %p] No more work need to be done!\n",
pthread_self());
pthread_mutex_unlock(&mutex);
}
}
int main()
{
pthread_t thread_1, thread_2;
pthread_create(&thread_1, NULL, thread_func, NULL);
pthread_create(&thread_2, NULL, thread_func, NULL);
printf("[main] Started working threads: %p, %p\n", thread_1, thread_2);
for (size_t i = 0; i < COUNT_MAX; ++i) {
pthread_mutex_lock(&mutex);
++counter;
pthread_mutex_unlock(&mutex);
pthread_cond_signal(&cond_var);
}
printf("Finished counting to %u\n", COUNT_MAX);
pthread_join(thread_1, NULL);
pthread_join(thread_2, NULL);
return 0;
}
And compile with command "musl-gcc -static main.c", it outputs:
[main] Started working threads: 0x7efc7f212f38, 0x7efc7f1eff38
[worker 0x7efc7f212f38] No more work need to be done!
[worker 0x7efc7f1eff38] No more work need to be done!
[worker 0x7efc7f1eff38] No more work need to be done!
[worker 0x7efc7f1eff38] No more work need to be done!
[worker 0x7efc7f212f38] Spurious wakeup occurred! Counter is 0!
William
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