linux-kernel - Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before

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Date:   Fri, 10 Jul 2020 11:42:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     linux-arch@...r.kernel.org, x86@...nel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-mm@...ck.org,
        Anton Blanchard <anton@...abs.org>
Subject: 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.