From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mathieu Desnoyers Subject: Re: [RFC PATCH 4/7] x86: use exit_lazy_tlb rather than membarrier_mm_sync_core_before_usermode Date: Thu, 16 Jul 2020 11:46:22 -0400 (EDT) Message-ID: <1494299304.15894.1594914382695.JavaMail.zimbra@efficios.com> 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> <1594868476.6k5kvx8684.astroid@bobo.none> <1594873644.viept6os6j.astroid@bobo.none> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: Received: from mail.efficios.com ([167.114.26.124]:41972 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728589AbgGPPqY (ORCPT ); Thu, 16 Jul 2020 11:46:24 -0400 In-Reply-To: <1594873644.viept6os6j.astroid@bobo.none> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Nicholas Piggin Cc: Anton Blanchard , Arnd Bergmann , linux-arch , linux-kernel , linux-mm , linuxppc-dev , Andy Lutomirski , Peter Zijlstra , x86 ----- On Jul 16, 2020, at 12:42 AM, Nicholas Piggin npiggin@gmail.com wrote: > I should be more complete here, especially since I was complaining > about unclear barrier comment :) > > > CPU0 CPU1 > a. user stuff 1. user stuff > b. membarrier() 2. enter kernel > c. smp_mb() 3. smp_mb__after_spinlock(); // in __schedule > d. read rq->curr 4. rq->curr switched to kthread > e. is kthread, skip IPI 5. switch_to kthread > f. return to user 6. rq->curr switched to user thread > g. user stuff 7. switch_to user thread > 8. exit kernel > 9. more user stuff > > What you're really ordering is a, g vs 1, 9 right? > > In other words, 9 must see a if it sees g, g must see 1 if it saw 9, > etc. > > Userspace does not care where the barriers are exactly or what kernel > memory accesses might be being ordered by them, so long as there is a > mb somewhere between a and g, and 1 and 9. Right? This is correct. Note that the accesses to user-space memory can be done either by user-space code or kernel code, it doesn't matter. However, in order to be considered as happening before/after either membarrier or the matching compiler barrier, kernel code needs to have causality relationship with user-space execution, e.g. user-space does a system call, or returns from a system call. In the case of io_uring, submitting a request or returning from waiting on request completion appear to provide this causality relationship. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com