linux: wakeup the target thread in kthread_park().

open-code sleepq_timeout() but without setting LW_STIMO.  fixes hang
in amdgpu GPU reset sequence, mirrors the linux semantics.


diff --git a/sys/external/bsd/drm2/linux/linux_kthread.c b/sys/external/bsd/drm2/linux/linux_kthread.c
index bb5c7d576afe..e1681c00320b 100644
--- a/sys/external/bsd/drm2/linux/linux_kthread.c
+++ b/sys/external/bsd/drm2/linux/linux_kthread.c
@@ -40,6 +40,7 @@ __KERNEL_RCSID(0, "$NetBSD$");
 #include <sys/lwp.h>
 #include <sys/mutex.h>
 #include <sys/specificdata.h>
+#include <sys/sleepq.h>
 
 #include <linux/kthread.h>
 
@@ -188,8 +189,20 @@ kthread_park(struct task_struct *T)
 	mutex_enter(&T->kt_lock);
 	KASSERT(!T->kt_shouldstop);
 	T->kt_shouldpark = true;
-	while (!T->kt_parked)
-		cv_wait(&T->kt_cv, &T->kt_lock);
+	lwp_t *l = T->kt_lwp;
+	if (l != curlwp) {
+		/* Open-coded sleepq_timeout() with no LW_STIMO. */
+		lwp_lock(l);
+		if (l->l_wchan == NULL) {
+			/* Somebody beat us to it. */
+			lwp_unlock(l);
+		} else {
+			lwp_unsleep(l, true);
+
+			while (!T->kt_parked)
+				cv_wait(&T->kt_cv, &T->kt_lock);
+		}
+	}
 	mutex_exit(&T->kt_lock);
 }