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 14:58:41 -0400 (EDT) Message-ID: <595582123.17106.1594925921537.JavaMail.zimbra@efficios.com> References: <20200710015646.2020871-1-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> <1494299304.15894.1594914382695.JavaMail.zimbra@efficios.com> <1370747990.15974.1594915396143.JavaMail.zimbra@efficios.com> 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]:59594 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726986AbgGPS6n (ORCPT ); Thu, 16 Jul 2020 14:58:43 -0400 In-Reply-To: <1370747990.15974.1594915396143.JavaMail.zimbra@efficios.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Nicholas Piggin , paulmck , Alan Stern Cc: Anton Blanchard , Arnd Bergmann , linux-arch , linux-kernel , linux-mm , linuxppc-dev , Andy Lutomirski , Peter Zijlstra , x86 ----- On Jul 16, 2020, at 12:03 PM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote: > ----- On Jul 16, 2020, at 11:46 AM, Mathieu Desnoyers > mathieu.desnoyers@efficios.com wrote: > >> ----- 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. > > Actually, sorry, the above is not quite right. It's been a while > since I looked into the details of membarrier. > > The smp_mb() at the beginning of membarrier() needs to be paired with a > smp_mb() _after_ rq->curr is switched back to the user thread, so the > memory barrier is between store to rq->curr and following user-space > accesses. > > The smp_mb() at the end of membarrier() needs to be paired with the > smp_mb__after_spinlock() at the beginning of schedule, which is > between accesses to userspace memory and switching rq->curr to kthread. > > As to *why* this ordering is needed, I'd have to dig through additional > scenarios from https://lwn.net/Articles/573436/. Or maybe Paul remembers ? Thinking further about this, I'm beginning to consider that maybe we have been overly cautious by requiring memory barriers before and after store to rq->curr. If CPU0 observes a CPU1's rq->curr->mm which differs from its own process (current) while running the membarrier system call, it necessarily means that CPU1 had to issue smp_mb__after_spinlock when entering the scheduler, between any user-space loads/stores and update of rq->curr. Requiring a memory barrier between update of rq->curr (back to current process's thread) and following user-space memory accesses does not seem to guarantee anything more than what the initial barrier at the beginning of __schedule already provides, because the guarantees are only about accesses to user-space memory. Therefore, with the memory barrier at the beginning of __schedule, just observing that CPU1's rq->curr differs from current should guarantee that a memory barrier was issued between any sequentially consistent instructions belonging to the current process on CPU1. Or am I missing/misremembering an important point here ? Thanks, Mathieu > > Thanks, > > Mathieu > > >> 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 > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com