diff --git a/sys/kern/kern_thread.c b/sys/kern/kern_thread.c
index 89f6137b6999..40652b6e85e3 100644
--- a/sys/kern/kern_thread.c
+++ b/sys/kern/kern_thread.c
@@ -981,7 +981,12 @@ void
 tidhash_add(struct thread *td)
 {
 	rw_wlock(&tidhash_lock);
-	LIST_INSERT_HEAD(TIDHASH(td->td_tid), td, td_hash);
+	thread_lock(td);
+	if ((td->td_flags & TDF_TIDHASH) == 0) {
+		LIST_INSERT_HEAD(TIDHASH(td->td_tid), td, td_hash);
+		td->td_flags |= TDF_TIDHASH;
+	}
+	thread_unlock(td);
 	rw_wunlock(&tidhash_lock);
 }
 
@@ -989,6 +994,11 @@ void
 tidhash_remove(struct thread *td)
 {
 	rw_wlock(&tidhash_lock);
-	LIST_REMOVE(td, td_hash);
+	thread_lock(td);
+	if ((td->td_flags & TDF_TIDHASH) != 0) {
+		LIST_REMOVE(td, td_hash);
+		td->td_flags &= ~TDF_TIDHASH;
+	}
+	thread_unlock(td);
 	rw_wunlock(&tidhash_lock);
 }
diff --git a/sys/sys/proc.h b/sys/sys/proc.h
index 9bce5691c20d..21fc29fab94c 100644
--- a/sys/sys/proc.h
+++ b/sys/sys/proc.h
@@ -353,7 +353,7 @@ do {									\
 #define	TDF_NEEDRESCHED	0x00010000 /* Thread needs to yield. */
 #define	TDF_NEEDSIGCHK	0x00020000 /* Thread may need signal delivery. */
 #define	TDF_NOLOAD	0x00040000 /* Ignore during load avg calculations. */
-#define	TDF_UNUSED19	0x00080000 /* Thread is sleeping on a umtx. */
+#define	TDF_TIDHASH	0x00080000 /* Thread is on hash table. */
 #define	TDF_THRWAKEUP	0x00100000 /* Libthr thread must not suspend itself. */
 #define	TDF_UNUSED21	0x00200000 /* --available-- */
 #define	TDF_SWAPINREQ	0x00400000 /* Swapin request due to wakeup. */