kern/thread: add comment about balancer threads

Balancer threads use their own version of thread_sleep because they have to
keep the run queue locked after pulling threads, otherwise remote balancers
might pull threads from a run queue right before a scheduling decision is
made, possibly putting the run queue in a state where only expired threads
remain.

Although this might be relaxed in the future, this is how it's currently
done, and a clear note in the main balancer function definitely helps against
breaking that part.

1 files changed, 2 insertions, 1 deletions

diff --git a/kern/thread.c b/kern/thread.c
index 1b9ab6f3..b01a3a50 100644
--- a/kern/thread.c
+++ b/kern/thread.c
@@ -1574,7 +1574,8 @@ thread_balance(void *arg)
 
         /*
          * This function may temporarily enable preemption and release the
-         * run queue lock.
+         * run queue lock, but on return, the lock must remain held until this
+         * balancer thread sleeps.
          */
         thread_sched_ts_balance(runq, &flags);
     }