On Sun, 12 Oct 2014, Qiaowei Ren wrote:
> +     /*
> +      * Go poke the address of the new bounds table in to the
> +      * bounds directory entry out in userspace memory.  Note:
> +      * we may race with another CPU instantiating the same table.
> +      * In that case the cmpxchg will see an unexpected
> +      * 'actual_old_val'.
> +      */
> +     ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry,
> +                                        expected_old_val, bt_addr);

This is fully preemptible non-atomic context, right?

So this wants a proper comment, why using
user_atomic_cmpxchg_inatomic() is the right thing to do here.

Thanks,

        tglx
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