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Message-ID: <20260728050227.GG27423@brightrain.aerifal.cx>
Date: Tue, 28 Jul 2026 01:02:28 -0400
From: Rich Felker <dalias@...c.org>
To: ShengYi Hung <aokblast@...eBSD.org>
Cc: musl@...ts.openwall.com
Subject: Re: [PATCH] detect deadlock when waiting itself
On Tue, Jul 28, 2026 at 12:49:30PM +0800, ShengYi Hung wrote:
> It is possible to call pthread_join(pthread_self()) from user. In the
> original implementation, it causes deadlock and never returns. In both
> glibc and FreeBSD libc implementation. It returns a EDEADLK immediately.
>
> This is found when running libcxx, which is built upon musl, testsuite.
> In thread.jthread/join.deadlock test, jthread::join will throw an
> exception if EDEADLK is returned. Thus can jump out from the deadlock.
> ---
> src/thread/pthread_join.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/src/thread/pthread_join.c b/src/thread/pthread_join.c
> index 17dae85d..b66c318c 100644
> --- a/src/thread/pthread_join.c
> +++ b/src/thread/pthread_join.c
> @@ -10,9 +10,14 @@ weak_alias(dummy1, __tl_sync);
> static int __pthread_timedjoin_np(pthread_t t, void **res, const struct timespec *at)
> {
> int state, cs, r = 0;
> + pthread_t self = __pthread_self();
> __pthread_testcancel();
> __pthread_setcancelstate(PTHREAD_CANCEL_DISABLE, &cs);
> if (cs == PTHREAD_CANCEL_ENABLE) __pthread_setcancelstate(cs, 0);
> + if (self == t) {
> + __pthread_setcancelstate(cs, 0);
> + return EDEADLK;
> + }
> while ((state = t->detach_state) && r != ETIMEDOUT && r != EINVAL) {
> if (state >= DT_DETACHED) a_crash();
> r = __timedwait_cp(&t->detach_state, state, CLOCK_REALTIME, at, 1);
> --
> 2.55.0
EDEADLK is a "may fail", not a "shall fail" condition. There should
not be a test asserting that it behaves this way. If it's part of the
contract of jthread::join that it detect attempts at self-join, it
should be checking this condition itself, not making nonportable
assumptions about pthread_join.
Rich
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