double call identical release when there is a race hitting

[zhong jiang <zhongjiang@xxxxxxxxxx>]

Hi

when I review the code, I find the following scenario will lead to a race ,
but I am not sure whether the real issue will hit or not.

cpu1                                                                      cpu2
exit_mmap                                               mmu_notifier_unregister
   __mmu_notifier_release                                 srcu_read_lock
            srcu_read_lock
            mm->ops->release(mn, mm)                 mm->ops->release(mn,mm)
           srcu_read_unlock                                         srcu_read_unlock


obviously,  the specified mm will call identical release function when
the related condition satisfy.  is it right?

Thanks
zhongjiang

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