percpu_down_read_trylock
if (!percpu_down_read_trylock(&hu->proto_lock))
return percpu_down_read_trylock(sb->s_writers.rw_sem + level - 1);
DEFINE_GUARD_COND(percpu_read, _try, percpu_down_read_trylock(_T))
return percpu_down_read_trylock(&cpu_hotplug_lock);