On 2019-08-31 11:28, Maxim Dounin wrote:
> Hello!
>
> On Fri, Aug 30, 2019 at 06:26:57PM -0700, Rian Hunter wrote:
>
>> On 2019-08-30 07:19, Maxim Dounin wrote:
>> > Yes, this is intentional.
>> >
>> > The first agument of the ngx_shmtx_create() function is a pointer
>> > to the ngx_shmtx_t structure, which is not expected to be shared
>> > between processes. Copy of the structure as created by fork() is
>> > enough.
>>
>> The POSIX sem_t member (called "sem") needs to reside in shared memory
>> if sem_wait()/sem_post() are to have an effect across processes.
>> Memory
>> copied across fork is not sufficient.
>
> No, fork() is explicitly documented to preserve semaphores
> (http://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html):
>
> : Any semaphores that are open in the parent process shall also be
> : open in the child process.

That's only true for semaphores created with sem_open(). For semaphores
created with sem_init(pshared=1) the caller is responsible for using
supplying memory that resides in a shared memory segment (e.g. created
from mm ap(flags=MAP_SHARED|...)).

See:
https://blog.superpat.com/2010/07/14/semaphores-on-linux-sem_init-vs-sem_open/

Here is a program that illustrates my point (also here:
https://gist.github.com/rianhunter/a0bd4c9e8ab550ecadbe2464995726a8):

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#include <errno.h>
#include <semaphore.h>
#include <time.h>
#include <unistd.h>

#include <sys/mman.h>
#include <sys/types.h>
#include <sys/wait.h>

int test_semaphore(sem_t *sem) {
pid_t pid;

pid = fork();

if (pid < 0) {
fprintf(stderr, "failed to fork: %s\n", strerror(errno));
return -1;
}

if (pid > 0) {
int status;

/* after posting, child should exit successfully */
sem_post(sem);

printf("Waiting for child to react to sem_post()...\n");
waitpid(pid, &status, 0);

if (!WIFEXITED(status) || WEXITSTATUS(status) > 1) {
return -1;
}

return !WEXITSTATUS(status);
} else {
int ret;
struct timespec to;

ret = clock_gettime(CLOCK_REALTIME, &to);
if (ret < 0) {
fprintf(stderr, "failed to clock_gettime(): %s\n",
strerror(errno));
return 255;
}

/* wait for sem_post() for 5 seconds */
to.tv_sec += 5;
ret = sem_timedwait(sem, &to);
if (ret < 0 && errno != ETIMEDOUT) {
fprintf(stderr, "failed to timedwait: %s\n",
strerror(errno));
return 255;
}
exit(!ret ? EXIT_SUCCESS : EXIT_FAILURE);
}

/* notreached */
}

int main() {
int ret;
sem_t stack_allocated, *mmap_shared;

sem_init(&stack_allocated, 1, 0);

ret = test_semaphore(&stack_allocated);
if (ret < 0) {
return EXIT_FAILURE;
}

if (!ret) {
printf("Stack-allocated semaphore across fork() is
broken\n");
} else {
printf("Stack-allocated semaphore across fork() works\n");
}

printf("\n");

mmap_shared = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE,
MAP_SHARED | MAP_ANONYMOUS, -1, 0);
if (mmap_shared == MAP_FAILED) {
return EXIT_FAILURE;
}

sem_init(mmap_shared, 1, 0);

ret = test_semaphore(mmap_shared);
if (ret < 0) {
return EXIT_FAILURE;
}

if (!ret) {
printf("Mapped semaphore across fork() is broken\n");
} else {
printf("Mapped semaphore across fork() works\n");
}

return EXIT_SUCCESS;
}
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