Re: [musl] Suggestion for thread safety

[Joakim Sindholt <opensource@zhasha.com>]

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On Sat, 26 Feb 2022 09:56:04 +0000, Lee Shallis <gb2985@gmail.com> wrote:
> On Wed, 23 Feb 2022 at 18:58, Markus Wichmann <nullplan@gmx.net> wrote:
> >
> > On Wed, Feb 23, 2022 at 12:30:43AM +0000, Lee Shallis wrote:
> > > think of the lock as the same as a mutex, just simpler,
> >
> > It isn't really simpler than a fast mutex, but a lot buggier.
> >
> There are no bugs, I made sure to test this after all, I deliberately
> created a situation where any bugs would show them selves, the only
> thing that was a potential problem I've now fixed (after doing some
> research to find out if 0 is ever a valid thread id), the concept
> remains the same, just with support for developer calling LockSiData
> twice:

Did I use it wrong then?

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

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

static void NoPause(void *ud)
{
    (void)ud;
}

typedef int dint;
typedef const char* LOCK;
void (*pauseCB)(void *) = NoPause;

static void LockSiData( LOCK **shared, LOCK *ptr )
{
    while ( *shared != ptr )
    {
        if ( !(*shared) )
            *shared = ptr;
        pauseCB( ptr );
    }
}

static dint FreeSiData( LOCK **shared, LOCK *ud )
{
    if ( *shared == ud )
    {
        *shared = NULL;
        return 0;
    }
    return EPERM;
}

static LOCK *l = 0;
static volatile int var = 0;

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

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

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