Date: Mon, 28 Sep 2015 18:21:21 -0400 From: Rich Felker <dalias@...c.org> To: musl@...ts.openwall.com Subject: Re: [PATCH 1/2] pthread: add missing memory barrier 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
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