Hello,
Greetings from me!
I am reading "4.3.4.2 A Volatile Solution" of perfbook, and come
across following summary:
To summarize, the volatile keyword can prevent load tearing and
store tearing in cases where the loads and stores are machine-sized
and properly aligned. It can also prevent load fusing, store fusing,
invented loads, and invented stores. ...
At first I thought it means accessing volatile, aligned and
machine-sized data is atomic operation, so I wrote a small test
program to test on a "64-bit" Linux server:
#include <pthread.h>
#include <stdio.h>
#include <stdatomic.h>
#include <stdint.h>
volatile uint64_t sum;
atomic_ullong atomic_sum;
void *thread(void *arg)
{
for (int i = 0; i < 100000; i++)
{
sum++;
atomic_fetch_add(&atomic_sum, 1);
}
return NULL;
}
int main()
{
pthread_t tid[4];
for (int i = 0; i < sizeof(tid) / sizeof(tid[0]); i++)
{
pthread_create(&tid[i], NULL, thread, NULL);
}
for (int i = 0; i < sizeof(tid) / sizeof(tid[0]); i++)
{
pthread_join(tid[i], NULL);
}
printf("sum=%llu,atomic_sum=%llu\n", sum, atomic_sum);
return 0;
}
But the result seems not:
$ gcc -pthread -O3 parallel.c -o parallel
$ ./parallel
sum=221785,atomic_sum=400000
$ gcc --version
gcc (GCC) 8.5.0 20210514 (Red Hat 8.5.0-22)
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
So my understanding is: volatile doesn't guarantee the atomic
operation for aligned, machine-sized data, and we can only use
atomic_xxx data types and related functions to guarantee atomic
operations. Is my understanding correct? Or I misunderstood volatile?
Thanks very much in advance!
Best Regards
Nan Xiao
