From 04193d590b390ec7a0592630f46d559ec6564ba1 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Tue, 7 Jun 2022 22:41:55 +0200 Subject: sched: Fix balance_push() vs __sched_setscheduler() The purpose of balance_push() is to act as a filter on task selection in the case of CPU hotplug, specifically when taking the CPU out. It does this by (ab)using the balance callback infrastructure, with the express purpose of keeping all the unlikely/odd cases in a single place. In order to serve its purpose, the balance_push_callback needs to be (exclusively) on the callback list at all times (noting that the callback always places itself back on the list the moment it runs, also noting that when the CPU goes down, regular balancing concerns are moot, so ignoring them is fine). And here-in lies the problem, __sched_setscheduler()'s use of splice_balance_callbacks() takes the callbacks off the list across a lock-break, making it possible for, an interleaving, __schedule() to see an empty list and not get filtered. Fixes: ae7927023243 ("sched: Optimize finish_lock_switch()") Reported-by: Jing-Ting Wu Signed-off-by: Peter Zijlstra (Intel) Tested-by: Jing-Ting Wu Link: https://lkml.kernel.org/r/20220519134706.GH2578@worktop.programming.kicks-ass.net --- kernel/sched/sched.h | 5 +++++ 1 file changed, 5 insertions(+) (limited to 'kernel/sched/sched.h') diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h index 01259611beb9..47b89a0fc6e5 100644 --- a/kernel/sched/sched.h +++ b/kernel/sched/sched.h @@ -1693,6 +1693,11 @@ queue_balance_callback(struct rq *rq, { lockdep_assert_rq_held(rq); + /* + * Don't (re)queue an already queued item; nor queue anything when + * balance_push() is active, see the comment with + * balance_push_callback. + */ if (unlikely(head->next || rq->balance_callback == &balance_push_callback)) return; -- cgit v1.2.1