From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: [PATCH 00/22] add support for Clang LTO Date: Thu, 25 Jun 2020 10:57:45 +0200 Message-ID: <20200625085745.GD117543@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: List-Post: List-Help: List-Unsubscribe: List-Subscribe: Content-Disposition: inline In-Reply-To: <20200625082433.GC117543@hirez.programming.kicks-ass.net> To: Nick Desaulniers Cc: Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" List-Id: linux-arch.vger.kernel.org On Thu, Jun 25, 2020 at 10:24:33AM +0200, Peter Zijlstra wrote: > On Thu, Jun 25, 2020 at 10:03:13AM +0200, Peter Zijlstra wrote: > > I'm sure Will will respond, but the basic issue is the trainwreck C11 > > made of dependent loads. > > > > Anyway, here's a link to the last time this came up: > > > > https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/ > > Another good read: > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > and having (partially) re-read that, I now worry intensily about things > like latch_tree_find(), cyc2ns_read_begin, __ktime_get_fast_ns(). > > It looks like kernel/time/sched_clock.c uses raw_read_seqcount() which > deviates from the above patterns by, for some reason, using a primitive > that includes an extra smp_rmb(). > > And this is just the few things I could remember off the top of my head, > who knows what else is out there. As an example, let us consider __ktime_get_fast_ns(), the critical bit is: seq = raw_read_seqcount_latch(&tkf->seq); tkr = tkf->base + (seq & 0x01); now = tkr->base; And we hard rely on that being a dependent load, so: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 MUL seq, sizeof(tk_read_base) ADD tkr, seq LOAD now, (tkr->base) Such that we obtain 'now' as a direct dependency on 'seq'. This ensures the loads are ordered. A compiler can wreck this by translating it into something like: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 CMP seq, 0 JE 1f ADD tkr, sizeof(tk_read_base) 1: LOAD now, (tkr->base) Because now the machine can speculate and load now before seq, breaking the ordering. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52072 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2390330AbgFYI6T (ORCPT ); Thu, 25 Jun 2020 04:58:19 -0400 Date: Thu, 25 Jun 2020 10:57:45 +0200 From: Peter Zijlstra Subject: Re: [PATCH 00/22] add support for Clang LTO Message-ID: <20200625085745.GD117543@hirez.programming.kicks-ass.net> References: <20200624203200.78870-1-samitolvanen@google.com> <20200624211540.GS4817@hirez.programming.kicks-ass.net> <20200625080313.GY4817@hirez.programming.kicks-ass.net> <20200625082433.GC117543@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200625082433.GC117543@hirez.programming.kicks-ass.net> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Nick Desaulniers Cc: Sami Tolvanen , Masahiro Yamada , Will Deacon , Greg Kroah-Hartman , "Paul E. McKenney" , Kees Cook , clang-built-linux , Kernel Hardening , linux-arch , Linux ARM , Linux Kbuild mailing list , LKML , linux-pci@vger.kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Message-ID: <20200625085745.MP7QIq3OcdG6JjBuEhhgiHOEQ4WS-fMTdzrBcjLclvU@z> On Thu, Jun 25, 2020 at 10:24:33AM +0200, Peter Zijlstra wrote: > On Thu, Jun 25, 2020 at 10:03:13AM +0200, Peter Zijlstra wrote: > > I'm sure Will will respond, but the basic issue is the trainwreck C11 > > made of dependent loads. > > > > Anyway, here's a link to the last time this came up: > > > > https://lore.kernel.org/linux-arm-kernel/20171116174830.GX3624@linux.vnet.ibm.com/ > > Another good read: > > https://lore.kernel.org/lkml/20150520005510.GA23559@linux.vnet.ibm.com/ > > and having (partially) re-read that, I now worry intensily about things > like latch_tree_find(), cyc2ns_read_begin, __ktime_get_fast_ns(). > > It looks like kernel/time/sched_clock.c uses raw_read_seqcount() which > deviates from the above patterns by, for some reason, using a primitive > that includes an extra smp_rmb(). > > And this is just the few things I could remember off the top of my head, > who knows what else is out there. As an example, let us consider __ktime_get_fast_ns(), the critical bit is: seq = raw_read_seqcount_latch(&tkf->seq); tkr = tkf->base + (seq & 0x01); now = tkr->base; And we hard rely on that being a dependent load, so: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 MUL seq, sizeof(tk_read_base) ADD tkr, seq LOAD now, (tkr->base) Such that we obtain 'now' as a direct dependency on 'seq'. This ensures the loads are ordered. A compiler can wreck this by translating it into something like: LOAD seq, (tkf->seq) LOAD tkr, tkf->base AND seq, 1 CMP seq, 0 JE 1f ADD tkr, sizeof(tk_read_base) 1: LOAD now, (tkr->base) Because now the machine can speculate and load now before seq, breaking the ordering.