_pthread_cancel() breaks the normal lock order of first locking the

joined and then the joiner thread. There isn't an easy (sane?) way
to make it use the correct order without introducing races involving
the target thread and finding which (active or dead) list it is on. So,
after locking the canceled thread it will try to lock the joined thread
and if it fails release the first lock and try again from the top.

Introduce a new function, _spintrylock, which is simply a wrapper arround
umtx_trylock(), to help accomplish this.

Approved by: re/blanket libthr

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/libthr/thread/thr_spinlock.c b/lib/libthr/thread/thr_spinlock.c
index ff9b9e0..0f9cb6b 100644
--- a/lib/libthr/thread/thr_spinlock.c
+++ b/lib/libthr/thread/thr_spinlock.c
@@ -69,6 +69,16 @@ _spinlock(spinlock_t *lck)
 	_spinlock_pthread(curthread, lck);
 }
 
+int
+_spintrylock(spinlock_t *lck)
+{
+	int error;
+	error = umtx_trylock((struct umtx *)lck, curthread->thr_id);
+	if (error != 0 && error != EBUSY)
+		abort();
+	return (error);
+}
+
 inline void
 _spinlock_pthread(pthread_t pthread, spinlock_t *lck)
 {