[ Upstream commit
f0498d2a54e7966ce23cd7c7ff42c64fa0059b07 ]
Kuyo reported sporadic failures on a sched_setaffinity() vs CPU
hotplug stress-test -- notably affine_move_task() remains stuck in
wait_for_completion(), leading to a hung-task detector warning.
Specifically, it was reported that stop_one_cpu_nowait(.fn =
migration_cpu_stop) returns false -- this stopper is responsible for
the matching complete().
The race scenario is:
CPU0 CPU1
// doing _cpu_down()
__set_cpus_allowed_ptr()
task_rq_lock();
takedown_cpu()
stop_machine_cpuslocked(take_cpu_down..)
<PREEMPT: cpu_stopper_thread()
MULTI_STOP_PREPARE
...
__set_cpus_allowed_ptr_locked()
affine_move_task()
task_rq_unlock();
<PREEMPT: cpu_stopper_thread()\>
ack_state()
MULTI_STOP_RUN
take_cpu_down()
__cpu_disable();
stop_machine_park();
stopper->enabled = false;
/>
/>
stop_one_cpu_nowait(.fn = migration_cpu_stop);
if (stopper->enabled) // false!!!
That is, by doing stop_one_cpu_nowait() after dropping rq-lock, the
stopper thread gets a chance to preempt and allows the cpu-down for
the target CPU to complete.
OTOH, since stop_one_cpu_nowait() / cpu_stop_queue_work() needs to
issue a wakeup, it must not be ran under the scheduler locks.
Solve this apparent contradiction by keeping preemption disabled over
the unlock + queue_stopper combination:
preempt_disable();
task_rq_unlock(...);
if (!stop_pending)
stop_one_cpu_nowait(...)
preempt_enable();
This respects the lock ordering contraints while still avoiding the
above race. That is, if we find the CPU is online under rq-lock, the
targeted stop_one_cpu_nowait() must succeed.
Apply this pattern to all similar stop_one_cpu_nowait() invocations.
Fixes:
6d337eab041d ("sched: Fix migrate_disable() vs set_cpus_allowed_ptr()")
Reported-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: "Kuyo Chang (張建文)" <Kuyo.Chang@mediatek.com>
Link: https://lkml.kernel.org/r/20231010200442.GA16515@noisy.programming.kicks-ass.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
* it.
*/
WARN_ON_ONCE(!pending->stop_pending);
+ preempt_disable();
task_rq_unlock(rq, p, &rf);
stop_one_cpu_nowait(task_cpu(p), migration_cpu_stop,
&pending->arg, &pending->stop_work);
+ preempt_enable();
return 0;
}
out:
complete = true;
}
+ preempt_disable();
task_rq_unlock(rq, p, rf);
-
if (push_task) {
stop_one_cpu_nowait(rq->cpu, push_cpu_stop,
p, &rq->push_work);
}
+ preempt_enable();
if (complete)
complete_all(&pending->done);
if (flags & SCA_MIGRATE_ENABLE)
p->migration_flags &= ~MDF_PUSH;
+ 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);
}
+ preempt_enable();
if (flags & SCA_MIGRATE_ENABLE)
return 0;
* Temporarily drop rq->lock such that we can wake-up the stop task.
* Both preemption and IRQs are still disabled.
*/
+ preempt_disable();
raw_spin_rq_unlock(rq);
stop_one_cpu_nowait(rq->cpu, __balance_push_cpu_stop, push_task,
this_cpu_ptr(&push_work));
+ preempt_enable();
/*
* At this point need_resched() is true and we'll take the loop in
* schedule(). The next pick is obviously going to be the stop task
double_unlock_balance(this_rq, src_rq);
if (push_task) {
+ preempt_disable();
raw_spin_rq_unlock(this_rq);
stop_one_cpu_nowait(src_rq->cpu, push_cpu_stop,
push_task, &src_rq->push_work);
+ preempt_enable();
raw_spin_rq_lock(this_rq);
}
}
busiest->push_cpu = this_cpu;
active_balance = 1;
}
- raw_spin_rq_unlock_irqrestore(busiest, flags);
+ preempt_disable();
+ raw_spin_rq_unlock_irqrestore(busiest, flags);
if (active_balance) {
stop_one_cpu_nowait(cpu_of(busiest),
active_load_balance_cpu_stop, busiest,
&busiest->active_balance_work);
}
+ preempt_enable();
}
} else {
sd->nr_balance_failed = 0;
*/
push_task = get_push_task(rq);
if (push_task) {
+ preempt_disable();
raw_spin_rq_unlock(rq);
stop_one_cpu_nowait(rq->cpu, push_cpu_stop,
push_task, &rq->push_work);
+ preempt_enable();
raw_spin_rq_lock(rq);
}
double_unlock_balance(this_rq, src_rq);
if (push_task) {
+ preempt_disable();
raw_spin_rq_unlock(this_rq);
stop_one_cpu_nowait(src_rq->cpu, push_cpu_stop,
push_task, &src_rq->push_work);
+ preempt_enable();
raw_spin_rq_lock(this_rq);
}
}