Messages in this thread | | | Date | Fri, 03 Nov 2017 14:45:45 +0000 | From | James Morse <> | Subject | get_online_cpus() from a preemptible() context (bug?) |
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Hi Thomas, Peter,
I'm trying to work out what stops a thread being pre-empted and migrated between calling get_online_cpus() and put_online_cpus().
According to __percpu_down_read(), its the pre-empt count: > * Due to having preemption disabled the decrement happens on > * the same CPU as the increment, avoiding the > * increment-on-one-CPU-and-decrement-on-another problem.
So this: > void cpus_read_lock(void) > { > percpu_down_read(&cpu_hotplug_lock); > + > + /* Can we migrated before we release this per-cpu lock? */ > + WARN_ON(preemptible()); > }
should never fire?
It does, some of the offenders: > kmem_cache_create > apply_workqueue_attrs > stop_machine > static_key_enable > lru_add_drain_all > __cpuhp_setup_state > kmem_cache_shrink > vmstat_shepherd > __cpuhp_state_add_instance
Trying to leave preempt disabled between the down/up leads to scheduling-while-atomic instead.
Can you point out what I've missed here?
Thanks,
James
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