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[110.174.173.27]) by smtp.gmail.com with ESMTPSA id nl8sm115886pjb.13.2020.07.13.09.37.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2020 09:37:57 -0700 (PDT) Date: Tue, 14 Jul 2020 02:37:52 +1000 From: Nicholas Piggin Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode To: Andy Lutomirski , Mathieu Desnoyers Cc: Anton Blanchard , Arnd Bergmann , linux-arch , linux-kernel , linux-mm , linuxppc-dev , Peter Zijlstra , x86 References: <20200710015646.2020871-1-npiggin@gmail.com> <20200710015646.2020871-5-npiggin@gmail.com> <1594613902.1wzayj0p15.astroid@bobo.none> <1594647408.wmrazhwjzb.astroid@bobo.none> <284592761.9860.1594649601492.JavaMail.zimbra@efficios.com> In-Reply-To: MIME-Version: 1.0 Message-Id: <1594657848.8og86nopq6.astroid@bobo.none> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Excerpts from Andy Lutomirski's message of July 14, 2020 1:48 am: > On Mon, Jul 13, 2020 at 7:13 AM Mathieu Desnoyers > wrote: >> >> ----- On Jul 13, 2020, at 9:47 AM, Nicholas Piggin npiggin@gmail.com wro= te: >> >> > Excerpts from Nicholas Piggin's message of July 13, 2020 2:45 pm: >> >> Excerpts from Andy Lutomirski's message of July 11, 2020 3:04 am: >> >>> Also, as it stands, I can easily see in_irq() ceasing to promise to >> >>> serialize. There are older kernels for which it does not promise to >> >>> serialize. And I have plans to make it stop serializing in the >> >>> nearish future. >> >> >> >> You mean x86's return from interrupt? Sounds fun... you'll konw where= to >> >> update the membarrier sync code, at least :) >> > >> > Oh, I should actually say Mathieu recently clarified a return from >> > interrupt doesn't fundamentally need to serialize in order to support >> > membarrier sync core. >> >> Clarification to your statement: >> >> Return from interrupt to kernel code does not need to be context seriali= zing >> as long as kernel serializes before returning to user-space. >> >> However, return from interrupt to user-space needs to be context seriali= zing. >> >=20 > Indeed, and I figured this out on the first read through because I'm > quite familiar with the x86 entry code. But Nick somehow missed this, > and Nick is the one who wrote the patch. >=20 > Nick, I think this helps prove my point. The code you're submitting > may well be correct, but it's unmaintainable. It's not. The patch I wrote for x86 is a no-op, it just moves existing x86 hook and code that's already there to a different name. Actually it's not quite a no-op, it't changes it to use hooks that are actually called in the right places. Because previously it was unmaintainable from point of view of generic mm -- it was not clear at all that the old one should have been called in other places where the mm goes non-lazy. Now with the exit_lazy_tlb hook, it can quite easily be spotted where it is missing. And x86 keeps their membarrier code in x86, and uses nice well defined lazy tlb mm hooks. > At the very least, this > needs a comment explaining, from the perspective of x86, *exactly* > what exit_lazy_tlb() is promising, why it's promising it, how it > achieves that promise, and what code cares about it. Or we could do > something with TIF flags and make this all less magical, although that > will probably end up very slightly slower. It's all documented there in existing comments plus the asm-generic exit_lazy_tlb specification added AFAIKS. Is the membarrier comment in finish_task_switch plus these ones not enough? Thanks, Nick