drm/amdkfd: fix a use after free race with mmu_notifer unregister

When using mmu_notifer_unregister_no_release() the caller must ensure
there is a SRCU synchronize before the mn memory is freed, otherwise use
after free races are possible, for instance:

     CPU0                                      CPU1
                                      invalidate_range_start
                                         hlist_for_each_entry_rcu(..)
 mmu_notifier_unregister_no_release(&p->mn)
 kfree(mn)
                                      if (mn->ops->invalidate_range_end)

The error unwind in amdkfd misses the SRCU synchronization.

amdkfd keeps the kfd_process around until the mm is released, so split the
flow to fully initialize the kfd_process and register it for find_process,
and with the notifier. Past this point the kfd_process does not need to be
cleaned up as it is fully ready.

The final failable step does a vm_mmap() and does not seem to impact the
kfd_process global state. Since it also cannot be undone (and already has
problems with undo if it internally fails), it has to be last.

This way we don't have to try to unwind the mmu_notifier_register() and
avoid the problem with the SRCU.

Along the way this also fixes various other error unwind bugs in the flow.

Fixes: 45102048 ("amdkfd: Add process queue manager module")
Link: https://lore.kernel.org/r/20190806231548.25242-10-jgg@ziepe.ca


Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>