On Mon, 11 Nov 2024 15:45:22 +0800, Nan Xiao wrote:
> Hello,
>
> Greetings from me!
Hi!
>
> 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!
Your code above does mostly the same things as Listings 5.1 and 5.2 in
Section 5.1 "Why Isn’t Concurrent Counting Trivial?".
"Atomic (or volatile) accesses" and "atomic increment operations"
are quite different.
Please read on!
Thanks, Akira
>
> Best Regards
> Nan Xiao
