Re: [musl] Suggestion for thread safety

[Lee Shallis <gb2985@gmail.com>]

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Seems the wait just wasn't long enough, at about 4 yields onwards the
results become consistent success, I've attached the file I did the
experiments in, I even tried it under -O3 and no exits were
encountered, so yes my method works, just needs a bit more wait time
for extreme cases

On Mon, 28 Feb 2022 at 08:48, Joakim Sindholt <opensource@zhasha.com> wrote:
>
> 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

[-- Attachment #2: lock.c --]
[-- Type: text/x-csrc, Size: 1725 bytes --]

#define _GNU_SOURCE
#include <stdbool.h>
#include <unistd.h>
#include <errno.h>
#include <time.h>
#include <sched.h>
#include <pthread.h>
#include <stdlib.h>
#include <stdio.h>

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

void LockSiData( LOCK *shared )
{
	pid_t tid = gettid();
	uint const yield = 4;
	uint i;
	/*
	struct timespec ts = {0};
	ts.tv_nsec = 1;
	*/

	while ( shared->tid != tid )
	{
		if ( !(shared->tid) )
			shared->tid = tid;
		//clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, 0);
		for ( i = 0; i < yield; ++i )
			sched_yield();
	}

	shared->num++;
}

void FreeSiData( LOCK *shared )
{
	pid_t tid = gettid();
	if ( shared->tid != tid )
		return;
	shared->num--;
	if ( shared->num )
		return;
	shared->tid = (pid_t)0;
	//sched_yield();
}

volatile uint quit = 0;
volatile uint data = 0;
LOCK lock = {0};

void* thread( void *ud )
{
	uint i;
	struct timespec ts = {0};
	ts.tv_nsec = 1;
	(void)ud;

	while ( quit < CLOCKS_PER_SEC )
	{
		LockSiData( &lock );

		i = data++;
		if (i != 0)
		{
			flockfile( stdout );
			printf("var = %u, expected 0\n", i);
			funlockfile( stdout );
			FreeSiData( &lock );
			exit(1);
		}

		/* Why is needed for the test? */
		clock_nanosleep(CLOCK_MONOTONIC, 0, &ts, 0);

		i = data--;
		if (i != 1)
		{
			flockfile( stdout );
			printf("var = %u, expected 1\n", i);
			funlockfile( stdout );
			FreeSiData( &lock );
			exit(1);
		}

		++quit;

		FreeSiData( &lock );
	}
	return ud;
}

int main()
{
	pthread_t pt;
	int i;

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