On 8/21/2012 9:03 PM, Stephen Boyd wrote: > Nothing stops a process from hotplugging in a CPU concurrently > with a sys_reboot() call. In such a situation we could have > ipi_cpu_stop() mark a cpu as 'offline' and _cpu_up() ignore the > fact that the CPU is not really offline and call the > CPU_UP_PREPARE notifier. When this happens stop_machine code will > complain that the cpu thread already exists and BUG_ON(). > > CPU0 CPU1 > > sys_reboot() > kernel_restart() > machine_restart() > machine_shutdown() > smp_send_stop() > ... ipi_cpu_stop() > set_cpu_online(1, false) > local_irq_disable() > while(1) > <PREEMPT> > cpu_up() > _cpu_up() > if (!cpu_online(1)) > __cpu_notify(CPU_UP_PREPARE...) > > cpu_stop_cpu_callback() > BUG_ON(stopper->thread) > > This is easily reproducible by hotplugging in and out in a tight > loop while also rebooting. > > Since the CPU is not really offline and hasn't gone through the > proper steps to be marked as such, let's mark the CPU as inactive. > This is just as easily testable as online and avoids any possibility > of _cpu_up() trying to bring the CPU back online when it never was > offline to begin with. > > Signed-off-by: Stephen Boyd <sb...@codeaurora.org> > --- > > Perhaps we can take the hotplug lock in the sys_reboot() case but I > don't think that actually fixes everything. For example, in cases > where machine_shutdown() is called from emergency_restart() we would > have to take the hotplug lock which doesn't really seem feasible.
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