Re: [PATCH v2 2/2] pthread: implement try/timed join variants

[Rich Felker <dalias@libc.org>]

On Sun, Oct 04, 2015 at 02:55:30PM -0500, Bobby Bingham wrote:
> ---
>  include/pthread.h         |  2 ++
>  src/thread/pthread_join.c | 29 ++++++++++++++++++++++-------
>  2 files changed, 24 insertions(+), 7 deletions(-)
> 
> diff --git a/include/pthread.h b/include/pthread.h
> index af70b73..3d2e0c4 100644
> --- a/include/pthread.h
> +++ b/include/pthread.h
> @@ -214,6 +214,8 @@ struct cpu_set_t;
>  int pthread_getaffinity_np(pthread_t, size_t, struct cpu_set_t *);
>  int pthread_setaffinity_np(pthread_t, size_t, const struct cpu_set_t *);
>  int pthread_getattr_np(pthread_t, pthread_attr_t *);
> +int pthread_tryjoin_np(pthread_t, void **);
> +int pthread_timedjoin_np(pthread_t, void **, const struct timespec *);
>  #endif
>  
>  #ifdef __cplusplus
> diff --git a/src/thread/pthread_join.c b/src/thread/pthread_join.c
> index 694d377..b041ff8 100644
> --- a/src/thread/pthread_join.c
> +++ b/src/thread/pthread_join.c
> @@ -5,18 +5,33 @@ int __munmap(void *, size_t);
>  void __pthread_testcancel(void);
>  int __pthread_setcancelstate(int, int *);
>  
> -int __pthread_join(pthread_t t, void **res)
> +int __pthread_tryjoin_np(pthread_t t, void **res)
>  {
> -	int tmp, cs;
> -	__pthread_testcancel();
> -	__pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
> -	if (cs == PTHREAD_CANCEL_ENABLE) __pthread_setcancelstate(cs, 0);
> -	while ((tmp = t->tid)) __timedwait_cp(&t->tid, tmp, 0, 0, 0);
> -	__pthread_setcancelstate(cs, 0);
> +	if (t->tid) return EBUSY;
>  	a_barrier();
>  	if (res) *res = t->result;
>  	if (t->map_base) __munmap(t->map_base, t->map_size);
>  	return 0;
>  }

This can spuriously fail when lack of memory synchronization causes
the caller not to see that the target thread has exited. However this
condition does not seem observable; calling pthread_tryjoin_np from
multiple threads where the calls could race is certainly UB, and
otherwise there should be no way to distinguish between the state of
just-before-exiting and already-exited. So I _think_ this is okay.

> +int __pthread_timedjoin_np(pthread_t t, void **res, const struct timespec *at)
> +{
> +	int tmp, cs, r = 0;
> +	__pthread_testcancel();
> +	__pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
> +	if (cs == PTHREAD_CANCEL_ENABLE) __pthread_setcancelstate(cs, 0);
> +	while ((tmp = t->tid) && r != ETIMEDOUT && r != EINVAL)
> +		r = __timedwait_cp(&t->tid, tmp, CLOCK_REALTIME, at, 0);
> +	__pthread_setcancelstate(cs, 0);
> +	if (r == ETIMEDOUT || r == EINVAL) return r;
> +	return __pthread_tryjoin_np(t, res);
> +}
> +
> +int __pthread_join(pthread_t t, void **res)
> +{
> +	return __pthread_timedjoin_np(t, res, 0);
> +}
> +
> +weak_alias(__pthread_tryjoin_np, pthread_tryjoin_np);
> +weak_alias(__pthread_timedjoin_np, pthread_timedjoin_np);
>  weak_alias(__pthread_join, pthread_join);

This all looks okay to me. I'll probably commit the patch as-is, as
long as nobody thinks the reasoning on the tryjoin issue above is
wrong.

Rich