[PATCH 1/2] pthread: add missing memory barrier

[PATCH 1/2] pthread: add missing memory barrier

[Bobby Bingham]

posix requires pthread_join to synchronize memory on success. in the
case that we call __timedwait_cp, we can rely on its futex_wake call
to synchronize memory, but we need a memory barrier for the case where
we don't call it.
---
 src/thread/pthread_join.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/thread/pthread_join.c b/src/thread/pthread_join.c
index 966b4ab..eb78e2f 100644
--- a/src/thread/pthread_join.c
+++ b/src/thread/pthread_join.c
@@ -11,7 +11,9 @@ int __pthread_join(pthread_t t, void **res)
 	__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);
+	if (!(tmp = t->tid)) a_barrier();
+	else do __timedwait_cp(&t->tid, tmp, 0, 0, 0);
+	while ((tmp = t->tid));
 	__pthread_setcancelstate(cs, 0);
 	if (res) *res = t->result;
 	if (t->map_base) __munmap(t->map_base, t->map_size);
-- 
2.5.3

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

[Bobby Bingham]

---
 include/pthread.h         |  2 ++
 src/thread/pthread_join.c | 34 +++++++++++++++++++++++++++-------
 2 files changed, 29 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 eb78e2f..9c3be08 100644
--- a/src/thread/pthread_join.c
+++ b/src/thread/pthread_join.c
@@ -5,19 +5,39 @@ int __munmap(void *, size_t);
 void __pthread_testcancel(void);
 int __pthread_setcancelstate(int, int *);
 
-int __pthread_join(pthread_t t, void **res)
+static int reclaim(pthread_t t, void **res)
 {
-	int tmp, cs;
+	if (res) *res = t->result;
+	if (t->map_base) __munmap(t->map_base, t->map_size);
+	return 0;
+}
+
+int __pthread_tryjoin_np(pthread_t t, void **res)
+{
+	if (t->tid) return EBUSY;
+	a_barrier();
+	return reclaim(t, res);
+}
+
+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);
 	if (!(tmp = t->tid)) a_barrier();
-	else do __timedwait_cp(&t->tid, tmp, 0, 0, 0);
-	while ((tmp = t->tid));
+	else do r = __timedwait_cp(&t->tid, tmp, CLOCK_REALTIME, at, 0);
+	while ((tmp = t->tid) && r != ETIMEDOUT && r != EINVAL);
 	__pthread_setcancelstate(cs, 0);
-	if (res) *res = t->result;
-	if (t->map_base) __munmap(t->map_base, t->map_size);
-	return 0;
+	if (r == ETIMEDOUT || r == EINVAL) return r;
+	return reclaim(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);
-- 
2.5.3

Re: [PATCH 1/2] pthread: add missing memory barrier

[Rich Felker]

On Mon, Sep 28, 2015 at 04:58:44PM -0500, Bobby Bingham wrote:
> posix requires pthread_join to synchronize memory on success. in the
> case that we call __timedwait_cp, we can rely on its futex_wake call
> to synchronize memory, but we need a memory barrier for the case where
> we don't call it.
> ---
>  src/thread/pthread_join.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/src/thread/pthread_join.c b/src/thread/pthread_join.c
> index 966b4ab..eb78e2f 100644
> --- a/src/thread/pthread_join.c
> +++ b/src/thread/pthread_join.c
> @@ -11,7 +11,9 @@ int __pthread_join(pthread_t t, void **res)
>  	__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);
> +	if (!(tmp = t->tid)) a_barrier();
> +	else do __timedwait_cp(&t->tid, tmp, 0, 0, 0);
> +	while ((tmp = t->tid));
>  	__pthread_setcancelstate(cs, 0);
>  	if (res) *res = t->result;
>  	if (t->map_base) __munmap(t->map_base, t->map_size);
> -- 

I think this works, but it depends on __timedwait_cp containing a
barrier. Is this something we should view as part of its contract or
as an implementation detail? We could punt on the question and just
put an unconditional a_barrier() after the loop above, but I'd kind of
like to think through the question while we're looking at it.

Rich