Specifically on an SMP kernel it was observed that if both the
processors are doing an exit1() via ast()->postsig()->sigexit()
then we will deadlock.
This happens because exit1() calls vmspace_exit() that in turn
calls pmap_invalidate_all(). This function tries to do a
smp_rendezvous() which blocks because the other processor is not
responding to IPIs - because it too is doing AST processing with
interrupts disabled.
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