diff --git a/src/sync/cond.rs b/src/sync/cond.rs
index 19f9868a19..171a35dc3a 100644
--- a/src/sync/cond.rs
+++ b/src/sync/cond.rs
@@ -61,8 +61,14 @@ impl Cond {
         self.wake(i32::MAX)
     }
     pub fn signal(&self) -> Result<(), Errno> {
-        self.broadcast()
-        //self.wake(1)
+        // POSIX requires pthread_cond_signal to wake AT LEAST ONE waiter that
+        // is currently waiting on the condition variable, but it must not
+        // wake all waiters (that is pthread_cond_broadcast semantics).
+        // Wake exactly one via FUTEX_WAKE with count=1. Using broadcast() here
+        // was a thundering-herd bug: every cond_signal woke every waiter on
+        // every CPU. Fixed 2026-07-02 (Red Bear OS multi-threading plan,
+        // Phase 0a).
+        self.wake(1)
     }
     pub fn clockwait(
         &self,