On 14/08/24 17:31, Daniel Vacek wrote:
CPU hangs were reported while offlining/onlining CPUs on s390.
Analyzing the vmcore data shows `stop_one_cpu_nowait()` in `affine_move_task()` can fail when racing with off-/on-lining resulting in a deadlock waiting for the pending migration stop work completion which is never done.
Fix this by gracefully handling such condition.
Fixes: 9e81889c7648 ("sched: Fix affine_move_task() self-concurrency") Cc: stable@vger.kernel.org Reported-by: Bill Peters wpeters@atpco.net Tested-by: Bill Peters wpeters@atpco.net Signed-off-by: Daniel Vacek neelx@redhat.com
kernel/sched/core.c | 21 +++++++++++++++++++-- 1 file changed, 19 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index f3951e4a55e5b..40a3c9ff74077 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2871,8 +2871,25 @@ static int affine_move_task(struct rq *rq, struct task_struct *p, struct rq_flag preempt_disable(); task_rq_unlock(rq, p, rf); if (!stop_pending) {
stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,&pending->arg, &pending->stop_work);
stop_pending =stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,&pending->arg, &pending->stop_work);/** The state resulting in this failure is not expected* at this point. At least report a WARNING to be able* to panic and further debug if reproduced.*/if (WARN_ON(!stop_pending)) {
I don't think it can happen when @p's CPU is being hot-unplugged, because takedown_cpu()->take_cpu_down() is done via stop_machine_cpuslocked(), so that task waking/running on that CPU implies the stopper is enabled.
I'm thinking there may be a window somewhere during hotplug, something like so?
CPU0 CPU1 CPU2 ---- ---- ---- secondary_start_kernel() set_cpu_online(true) wake_up_process(p) WRITE_ONCE(p->__state, TASK_WAKING); set_task_cpu(p, CPU1) // CPU1 is prev CPU ttwu_queue_wakelist(); affine_move_task(p) stop_one_cpu_nowait() // FAILS
cpu_startup_entry(CPUHP_AP_ONLINE_IDLE); cpuhp_online_idle() stop_machine_unpark(); // <-- too late