Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode

On Fri, Jul 10, 2020 at 11:56:43AM +1000, Nicholas Piggin wrote:

And get rid of the generic sync_core_before_usermode facility.

This helper is the wrong way around I think. The idea that membarrier
state requires a core sync before returning to user is the easy one
that does not need hiding behind membarrier calls. The gap in core
synchronization due to x86's sysret/sysexit and lazy tlb mode, is the
tricky detail that is better put in x86 lazy tlb code.

Consider if an arch did not synchronize core in switch_mm either, then
membarrier_mm_sync_core_before_usermode would be in the wrong place
but arch specific mmu context functions would still be the right place.
There is also a exit_lazy_tlb case that is not covered by this call, which
could be a bugs (kthread use mm the membarrier process's mm then context
switch back to the process without switching mm or lazy mm switch).

This makes lazy tlb code a bit more modular.

Hurmph, I know I've been staring at this at some point. I think I meant
to have a TIF to force the IRET path in the case of MEMBAR_SYNC_CORE.
But I was discouraged by amluto.