FIFO implementation, deadlock - c

Question

#include <math.h>
#include <pthread.h>
#include <stddef.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define MIN(x, y) ((x) < (y) ? (x) : (y))
#define TASK_LENGHT pow(10, 5) /* 0.1s */
#define TASK_COUNT 100
#define THREADS_CAP 1
struct fifo {
size_t out;
size_t in;
size_t size;
size_t len;
pthread_spinlock_t lock;
char data[1];
};
struct fifo *
fifo_alloc(size_t size)
{
struct fifo *fifo;
fifo = malloc(offsetof(struct fifo, data) + size);
fifo->out = fifo->in = 0;
fifo->size = size;
fifo->len = 0;
pthread_spin_init(&fifo->lock, PTHREAD_PROCESS_PRIVATE);
return fifo;
}
void
fifo_free(struct fifo *fifo)
{
pthread_spin_destroy(&fifo->lock);
free(fifo);
}
size_t
fifo_in(struct fifo *fifo, const void *data, size_t len)
{
size_t total, remaining, l;
pthread_spin_lock(&fifo->lock);
total = remaining = MIN(len, fifo->size - fifo->len);
while (remaining > 0) {
l = MIN(remaining, fifo->size - fifo->in);
memcpy(fifo->data + fifo->in, data, l);
fifo->in = (fifo->in + l) % fifo->size;
fifo->len += l;
data = (char *)data + l;
remaining -= l;
}
pthread_spin_unlock(&fifo->lock);
return total;
}
size_t
fifo_out(struct fifo *fifo, void *data, size_t len)
{
size_t total, remaining, out, l;
pthread_spin_lock(&fifo->lock);
total = remaining = MIN(len, fifo->len);
while (remaining > 0) {
l = MIN(remaining, fifo->size - fifo->out);
memcpy(data, fifo->data + fifo->out, l);
fifo->out = (fifo->out + l) % fifo->size;
data = (char *)data + l;
remaining -= l;
}
fifo->len -= total;
pthread_spin_unlock(&fifo->lock);
return total;
}
int
fifo_is_empty(struct fifo *fifo)
{
int empty;
pthread_spin_lock(&fifo->lock);
empty = fifo->len == 0;
pthread_spin_unlock(&fifo->lock);
return empty;
}
void *
process_task(void *datap)
{
struct fifo *tasks = datap;
int data;
while (1) {
puts("it blocks there");
if (!fifo_is_empty(tasks)) {
puts("this code never execute");
fifo_out(tasks, &data, sizeof(data));
if (data == -1) pthread_exit(0);
printf("%lu => %d\n", pthread_self(), data);
usleep(TASK_LENGHT);
}
puts("this code never execute");
}
}
int
main(void) {
pthread_t threads[THREADS_CAP];
struct fifo *tasks;
int data;
size_t i;
tasks = fifo_alloc(128);
for (i = 0; i < THREADS_CAP; i++)
pthread_create(threads + i, NULL, process_task, &tasks);
for (i = 0; i < TASK_COUNT; i++)
fifo_in(tasks, &i, sizeof(i));
for (i = 0; i < THREADS_CAP; i++) {
data = -1;
fifo_in(tasks, &data, sizeof(i));
}
puts("wait for threads to process all tasks");
for (i = 0; i < THREADS_CAP; i++) {
pthread_join(threads[i], NULL);
}
fifo_free(tasks);
}
I debugged my program and it turned out that it blocks at the pthread_spin_lock in fifo_is_empty. I was trying to recreate this problem with the minimum code needed like this:
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#define THREADS_CAP 10000
struct test {
pthread_spinlock_t lock;
};
pthread_barrier_t barrier;
int counter = 0;
void *
foo(void *data) {
struct test *test = data;
pthread_barrier_wait(&barrier);
pthread_spin_lock(&test->lock);
counter++;
pthread_spin_unlock(&test->lock);
return 0;
}
int main() {
struct test *test;
pthread_t threads[THREADS_CAP];
size_t i;
test = malloc(sizeof(test));
pthread_barrier_init(&barrier, NULL, THREADS_CAP);
pthread_spin_init(&test->lock, PTHREAD_PROCESS_PRIVATE);
for (i = 0; i < THREADS_CAP; i++)
pthread_create(threads + i, NULL, foo, test);
for (i = 0; i < THREADS_CAP; i++)
pthread_join(threads[i], NULL);
printf("%d\n", counter);
pthread_spin_destroy(&test->lock);
pthread_barrier_destroy(&barrier);
return 0;
}
but i was not able to. I just works for some reason in contrast to my previous code.
Does anyone have an idea what is causing there problem and how to fix it?

Answer 1

The problem took me a while to sort out, but I arrived there through working on a minimal reproducible example. I ultimately cut out everything related to managing the FIFO data, some unneeded functions, and all of the looping, but I found that the failure behavior did not manifest if I changed the definition of struct fifo.
Finally, I realized that the problem is here:
struct fifo *tasks;
// ...
pthread_create(threads + i, NULL, process_task, &tasks);
in conjunction with
void *process_task(void *datap) {
struct fifo *tasks = datap;
Bottom line: you are passing the address of your tasks pointer, a pointer to a pointer, but then your thread function tries to interpret it as a single pointer. Undefined behavior results.
The correction is to pass tasks itself to the thread, not &tasks:
pthread_create(threads + i, NULL, process_task, tasks);
Your program has other problems, but that resolves the one you asked about.