[RFC/PATCH] ARM: smp: Fix cpu_up() racing with sys_reboot

[Stephen Boyd]

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 <sboyd at 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.

Any comments on this patch?

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