From 515e3996a4c26e7f955c13b3b19522a2c8642af9 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Sun, 17 May 2026 07:43:16 -1000 Subject: [PATCH] sched_ext: Fix deadlock between scx_root_disable() and concurrent forks scx_root_disable() enters SCX_DISABLING before it grabs scx_enable_mutex to clear __scx_switched_all and scx_switching_all. task_should_scx() short-circuits on DISABLING, so forks in that window land on fair while next_active_class() still skips fair - the new tasks stall. This can deadlock the disable path itself: scx_alloc_and_add_sched() runs under scx_enable_mutex and creates a helper kthread; if that new kthread is one of the stalled fair tasks, the mutex holder waits forever and scx_root_disable() can never make progress. Only sub-sched support exposes this, since sub-sched enables are the only path where scx_alloc_and_add_sched() can race the root's disable. Move the DISABLING check after @scx_switching_all. @scx_switching_all serves as a proxy for __scx_switched_all, so while it's set, forks keep going to scx. Once cleared, DISABLING applies normally. v2: Reword in-source comment and description. (Andrea) Fixes: 337ec00b1d9c ("sched_ext: Implement cgroup sub-sched enabling and disabling") Signed-off-by: Tejun Heo Reviewed-by: Andrea Righi --- kernel/sched/ext.c | 22 +++++++++++++++++++++- 1 file changed, 21 insertions(+), 1 deletion(-) diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c index a6d0a93d8174..547ca398f646 100644 --- a/kernel/sched/ext.c +++ b/kernel/sched/ext.c @@ -4946,10 +4946,30 @@ static const struct kset_uevent_ops scx_uevent_ops = { */ bool task_should_scx(int policy) { - if (!scx_enabled() || unlikely(scx_enable_state() == SCX_DISABLING)) + /* if disabled, nothing should be on it */ + if (!scx_enabled()) return false; + + /* scx is taking over all SCHED_OTHER and SCHED_EXT tasks */ if (READ_ONCE(scx_switching_all)) return true; + + /* + * scx is tearing down - keep new SCHED_EXT tasks out. + * + * Must come after scx_switching_all test, which serves as a proxy + * for __scx_switched_all. While __scx_switched_all is set, we must + * return true via the branch above: a fork routed to fair would + * stall because next_active_class() skips fair. + * + * This can develop into a deadlock - scx holds scx_enable_mutex across + * kthread_create() in scx_alloc_and_add_sched(); if the new kthread is + * the stalled task, the disable path can never grab the mutex to clear + * scx_switching_all. + */ + if (unlikely(scx_enable_state() == SCX_DISABLING)) + return false; + return policy == SCHED_EXT; }