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From: Andy Lutomirski <luto@kernel.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Zijlstra <peterz@infradead.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
	Linux-MM <linux-mm@kvack.org>, Anton Blanchard <anton@ozlabs.org>
Subject: Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode
Date: Sat, 28 Nov 2020 09:55:37 -0800	[thread overview]
Message-ID: <CALCETrXYkbuJJnDv9ijfT+5tLQ2FOvvN1Ugoh5NwOy+zHp9HXA@mail.gmail.com> (raw)
In-Reply-To: <20201128160141.1003903-3-npiggin@gmail.com>

On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> And get rid of the generic sync_core_before_usermode facility. This is
> functionally a no-op in the core scheduler code, but it also catches
>
> 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.

I have a couple of membarrier fixes that I want to send out today or
tomorrow, and they might eliminate the need for this patch.  Let me
think about this a little bit.  I'll cc you.  The existing code is way
to subtle and the comments are far too confusing for me to be quickly
confident about any of my conclusions :)

WARNING: multiple messages have this Message-ID (diff)
From: Andy Lutomirski <luto@kernel.org>
To: Nicholas Piggin <npiggin@gmail.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
	Arnd Bergmann <arnd@arndb.de>,
	Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	linuxppc-dev <linuxppc-dev@lists.ozlabs.org>
Subject: Re: [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode
Date: Sat, 28 Nov 2020 09:55:37 -0800	[thread overview]
Message-ID: <CALCETrXYkbuJJnDv9ijfT+5tLQ2FOvvN1Ugoh5NwOy+zHp9HXA@mail.gmail.com> (raw)
In-Reply-To: <20201128160141.1003903-3-npiggin@gmail.com>

On Sat, Nov 28, 2020 at 8:02 AM Nicholas Piggin <npiggin@gmail.com> wrote:
>
> And get rid of the generic sync_core_before_usermode facility. This is
> functionally a no-op in the core scheduler code, but it also catches
>
> 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.

I have a couple of membarrier fixes that I want to send out today or
tomorrow, and they might eliminate the need for this patch.  Let me
think about this a little bit.  I'll cc you.  The existing code is way
to subtle and the comments are far too confusing for me to be quickly
confident about any of my conclusions :)

  reply	other threads:[~2020-11-28 22:14 UTC|newest]

Thread overview: 104+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-28 16:01 [PATCH 0/8] shoot lazy tlbs Nicholas Piggin
2020-11-28 16:01 ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 1/8] lazy tlb: introduce exit_lazy_tlb Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  0:38   ` Andy Lutomirski
2020-11-29  0:38     ` Andy Lutomirski
2020-11-29  0:38     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
2020-12-02  2:49       ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 2/8] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-28 17:55   ` Andy Lutomirski [this message]
2020-11-28 17:55     ` Andy Lutomirski
2020-11-28 17:55     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
2020-12-02  2:49       ` Nicholas Piggin
2020-12-03  5:09       ` Andy Lutomirski
2020-12-03  5:09         ` Andy Lutomirski
2020-12-03  5:09         ` Andy Lutomirski
2020-12-05  8:00         ` Nicholas Piggin
2020-12-05  8:00           ` Nicholas Piggin
2020-12-05 16:11           ` Andy Lutomirski
2020-12-05 16:11             ` Andy Lutomirski
2020-12-05 23:14             ` Nicholas Piggin
2020-12-05 23:14               ` Nicholas Piggin
2020-12-06  0:36               ` Andy Lutomirski
2020-12-06  0:36                 ` Andy Lutomirski
2020-12-06  0:36                 ` Andy Lutomirski
2020-12-06  3:59                 ` Nicholas Piggin
2020-12-06  3:59                   ` Nicholas Piggin
2020-12-11  0:11                   ` Andy Lutomirski
2020-12-11  0:11                     ` Andy Lutomirski
2020-12-11  0:11                     ` Andy Lutomirski
2020-12-14  4:07                     ` Nicholas Piggin
2020-12-14  4:07                       ` Nicholas Piggin
2020-12-14  5:53                       ` Nicholas Piggin
2020-12-14  5:53                         ` Nicholas Piggin
2020-11-30 14:57   ` Mathieu Desnoyers
2020-11-30 14:57     ` Mathieu Desnoyers
2020-11-30 14:57     ` Mathieu Desnoyers
2020-11-28 16:01 ` [PATCH 3/8] x86: remove ARCH_HAS_SYNC_CORE_BEFORE_USERMODE Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 4/8] lazy tlb: introduce lazy mm refcount helper functions Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 5/8] lazy tlb: allow lazy tlb mm switching to be configurable Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  0:36   ` Andy Lutomirski
2020-11-29  0:36     ` Andy Lutomirski
2020-11-29  0:36     ` Andy Lutomirski
2020-12-02  2:49     ` Nicholas Piggin
2020-12-02  2:49       ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 6/8] lazy tlb: shoot lazies, a non-refcounting lazy tlb option Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-29  3:54   ` Andy Lutomirski
2020-11-29  3:54     ` Andy Lutomirski
2020-11-29  3:54     ` Andy Lutomirski
2020-11-29 20:16     ` Andy Lutomirski
2020-11-29 20:16       ` Andy Lutomirski
2020-11-29 20:16       ` Andy Lutomirski
2020-11-30  9:25       ` Peter Zijlstra
2020-11-30  9:25         ` Peter Zijlstra
2020-11-30 18:31       ` Andy Lutomirski
2020-11-30 18:31         ` Andy Lutomirski
2020-11-30 18:31         ` Andy Lutomirski
2020-12-01 21:27         ` Will Deacon
2020-12-01 21:27           ` Will Deacon
2020-12-01 21:50           ` Andy Lutomirski
2020-12-01 21:50             ` Andy Lutomirski
2020-12-01 21:50             ` Andy Lutomirski
2020-12-01 23:04             ` Will Deacon
2020-12-01 23:04               ` Will Deacon
2020-12-02  3:47         ` Nicholas Piggin
2020-12-02  3:47           ` Nicholas Piggin
2020-12-03  5:05           ` Andy Lutomirski
2020-12-03  5:05             ` Andy Lutomirski
2020-12-03  5:05             ` Andy Lutomirski
2020-12-03 17:03         ` Alexander Gordeev
2020-12-03 17:03           ` Alexander Gordeev
2020-12-03 17:14           ` Andy Lutomirski
2020-12-03 17:14             ` Andy Lutomirski
2020-12-03 18:33             ` Alexander Gordeev
2020-12-03 18:33               ` Alexander Gordeev
2020-11-30  9:26     ` Peter Zijlstra
2020-11-30  9:26       ` Peter Zijlstra
2020-11-30  9:30     ` Peter Zijlstra
2020-11-30  9:30       ` Peter Zijlstra
2020-11-30  9:34       ` Peter Zijlstra
2020-11-30  9:34         ` Peter Zijlstra
2020-12-02  3:09     ` Nicholas Piggin
2020-12-02  3:09       ` Nicholas Piggin
2020-12-02 11:17   ` Peter Zijlstra
2020-12-02 11:17     ` Peter Zijlstra
2020-12-02 12:45     ` Peter Zijlstra
2020-12-02 12:45       ` Peter Zijlstra
2020-12-02 14:19   ` Peter Zijlstra
2020-12-02 14:19     ` Peter Zijlstra
2020-12-02 14:38     ` Andy Lutomirski
2020-12-02 14:38       ` Andy Lutomirski
2020-12-02 16:29       ` Peter Zijlstra
2020-12-02 16:29         ` Peter Zijlstra
2020-11-28 16:01 ` [PATCH 7/8] powerpc: use lazy mm refcount helper functions Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin
2020-11-28 16:01 ` [PATCH 8/8] powerpc/64s: enable MMU_LAZY_TLB_SHOOTDOWN Nicholas Piggin
2020-11-28 16:01   ` Nicholas Piggin

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