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Date: Mon, 28 Feb 2022 09:48:14 +0100
From: Joakim Sindholt <opensource@zhasha.com>
To: musl@lists.openwall.com
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Subject: Re: [musl] Suggestion for thread safety

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On Sun, 27 Feb 2022 23:32:47 +0000, Lee Shallis <gb2985@gmail.com> wrote:
> Yes, as I mentioned before, pauseCB is supposed to have it's pointer
> be changed by the developer, in other words you forgot to plugin a
> pthreads compatible call prior to your threads starting, considering
> you made that mistake I suppose it is a good thing I since switched to
> a non-redirectable pointer:

How many times do you want to do this?

zhasha@wirbelwind /home/zhasha ; gcc -D_GNU_SOURCE -lpthread buglock.c
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
var = 2, expected 1
zhasha@wirbelwind /home/zhasha ; ./a.out
var = 1, expected 0
var = 2, expected 1

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#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
#include <stdint.h>
#include <errno.h>
#include <unistd.h>

typedef int dint;
typedef pid_t WHICH;
typedef struct _LOCK LOCK;
struct _LOCK { uint num; WHICH tid; };

WHICH Which() { return gettid(); }
void Pause() { pthread_yield(); }

void LockSiData( LOCK *shared )
{
    WHICH tid = Which();
    while ( shared->tid != tid )
    {
        if ( !(shared->tid) )
            shared->tid = tid;
        Pause();
    }
    shared->num++;
}

dint FreeSiData( LOCK *shared )
{
    if ( shared->tid == Which() )
    {
        shared->num--;
        if ( !(shared->num) )
            shared->tid = (WHICH)0;
        return 0;
    }
    return EACCES;
}

static LOCK l;
static volatile int var = 0;

static void *thread(void *arg)
{
    int i;
    while (1) {
        LockSiData(&l);
        i = var++;
        if (i != 0) {
            printf("var = %d, expected 0\n", i);
            exit(1);
        }
        clock_nanosleep(CLOCK_MONOTONIC, 0, &(struct timespec){.tv_sec = 0, .tv_nsec = 1}, 0);
        i = var--;
        if (i != 1) {
            printf("var = %d, expected 1\n", i);
            exit(1);
        }
        FreeSiData(&l);
    }
    return 0;
}

int main(void)
{
    pthread_t pt;
    int i;

    for (i = 0; i < 2; i++) {
        if ((errno = pthread_create(&pt, 0, thread, 0)) != 0) {
            printf("pthread_create failed: %m\n");
            return 1;
        }
    }
    pthread_exit(0);
}

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