From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753247AbdJTMD5 convert rfc822-to-8bit (ORCPT ); Fri, 20 Oct 2017 08:03:57 -0400 Received: from mga06.intel.com ([134.134.136.31]:1591 "EHLO mga06.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753223AbdJTMDz (ORCPT ); Fri, 20 Oct 2017 08:03:55 -0400 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.43,405,1503385200"; d="scan'208";a="1233137619" From: "Reshetova, Elena" To: "'Peter Zijlstra'" CC: "'Thomas Gleixner'" , "'linux-kernel@vger.kernel.org'" , "'linux-fsdevel@vger.kernel.org'" , "'gregkh@linuxfoundation.org'" , "'viro@zeniv.linux.org.uk'" , "'tj@kernel.org'" , "'mingo@redhat.com'" , "'hannes@cmpxchg.org'" , "'lizefan@huawei.com'" , "'acme@kernel.org'" , "'alexander.shishkin@linux.intel.com'" , "'eparis@redhat.com'" , "'akpm@linux-foundation.org'" , "'arnd@arndb.de'" , "'luto@kernel.org'" , "'keescook@chromium.org'" , "'dvhart@infradead.org'" , "'ebiederm@xmission.com'" Subject: RE: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t Thread-Topic: [PATCH 14/15] futex: convert futex_pi_state.refcount to refcount_t Thread-Index: AQHTIYrzZIHADGwHSEu0EVkr0E9TAqKflyIAgAAhQgCAAAFJAIAAI4yQgAADB+CAAAkkgIAAGdsA///3mwCAABRlgIAASW8AgAQ1RNCAAAjqgIAQBuywgDhVnzA= Date: Fri, 20 Oct 2017 12:03:48 +0000 Message-ID: <2236FBA76BA1254E88B949DDB74E612B802B3903@IRSMSX102.ger.corp.intel.com> References: <20170901093852.it4d4bxoy2lmojrk@hirez.programming.kicks-ass.net> <2236FBA76BA1254E88B949DDB74E612B6FF6347F@IRSMSX102.ger.corp.intel.com> <20170901123415.s3fxlyeyourz47av@hirez.programming.kicks-ass.net> <2236FBA76BA1254E88B949DDB74E612B6FF63506@IRSMSX102.ger.corp.intel.com> <20170901133644.jf57pwuaep6zirxz@hirez.programming.kicks-ass.net> <2236FBA76BA1254E88B949DDB74E612B6FF6369D@IRSMSX102.ger.corp.intel.com> <20170901191234.ghybbmpm73miwmkp@hirez.programming.kicks-ass.net> <2236FBA76BA1254E88B949DDB74E612B6FF63ED7@IRSMSX102.ger.corp.intel.com> <20170904120009.ah2qu3lbgdqdgz6i@hirez.programming.kicks-ass.net> <2236FBA76BA1254E88B949DDB74E612B6FF6A282@IRSMSX102.ger.corp.intel.com> In-Reply-To: <2236FBA76BA1254E88B949DDB74E612B6FF6A282@IRSMSX102.ger.corp.intel.com> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: dlp-product: dlpe-windows dlp-version: 11.0.0.116 dlp-reaction: no-action x-ctpclassification: CTP_IC x-titus-metadata-40: eyJDYXRlZ29yeUxhYmVscyI6IiIsIk1ldGFkYXRhIjp7Im5zIjoiaHR0cDpcL1wvd3d3LnRpdHVzLmNvbVwvbnNcL0ludGVsMyIsImlkIjoiZjgzZTU1MGEtZWQxMi00M2I1LTg0YzAtNzEzZWY5MDExOGE4IiwicHJvcHMiOlt7Im4iOiJDVFBDbGFzc2lmaWNhdGlvbiIsInZhbHMiOlt7InZhbHVlIjoiQ1RQX0lDIn1dfV19LCJTdWJqZWN0TGFiZWxzIjpbXSwiVE1DVmVyc2lvbiI6IjE3LjIuNS4xOCIsIlRydXN0ZWRMYWJlbEhhc2giOiJjTjdtTHc3T1kzeFhsRzZmVWxYQUUxVlwvRUJcL1VETldUVEM3MW01YlwvMFd4UHNcLzlnWFRLeUM5RHJpazRydXFoYiJ9 x-originating-ip: [163.33.239.181] Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 8BIT MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Since I am not really sure what to do with this futex patch, I will drop it from the new series that I am about to send now. Please let me know what exactly should I do with this patch, as I wrote previously I really don't understand. Best Regards, Elena. > Sorry for delayed reply. > > > On Mon, Sep 04, 2017 at 10:31:54AM +0000, Reshetova, Elena wrote: > > > > > But can they make "fast" implementation on ARM that would give stronger > > > > > memory guarantees? > > > > > > > > Whatever for? > > > > > > Well, maybe just by default when arch.-specific implementation is > > > done. But I was just trying to speculate to understand. I will resend > > > this one with new comment added. > > > > So the generic lib/refcount.c already has weak ordering. It doesn't make > > sense for an arch specific implementation (on a weakly ordered machine) > > to provide stronger guarantees (it would make things slower). > > Thank you for explaining this! Helps to understand a lot. > > > > The weaker ordering of the refcount_t primitives is sufficient if we're > > talking pure refcounts. If for some reason code relies on stronger > > ordering there _SHOULD_ be a comment with describing the additional > > ordering requirements. > > > > But that's a fairly big 'should'. I can well imagine the comment not > > being there. In fact, see below. > > > > > Still not sure if I need to resend the whole series with updated > > > commits or break this up by individual patches further for the > > > separate merges. > > > > I've yet to look at the ones targeted at subsystems I do, I'm forever > > and terminally behind on review :/ > > > > I called out the issue on futex in particular because it is fairly > > tricky code that. > > > > Now Thomas would like you to mention the fact that refcount_t doesn't > > provide the exact same ordering as the atomic_t usages it replaces and > > I think it would be good if you could hand-wave an argument on why the > > futex code doesn't care. > > I think I can mention the ordering differences on all yet-to-be-merged > patches to make sure maintainers are aware. The problem with concrete > cases is that I don't usually have enough knowledge of code to understand > for sure where it would matter or not. Previously I was even under impression > that it should not matter at all for the variables that we are converting since > they are classical refcounters, but your examples clearly show that it is not > *always* the case (but I think it is the case for most of patches). > So, I am a bit confused on how to approach this. > Either just put a statement to all patches and rely that maintainers certainly > know their code and can catch these things or do an analysis myself, but > then I would need a bit of guidance on what is the reasonable heuristics on > how check each refcounter. This goes really beyond my current > kernel knowledge, but I am happy to learn if somebody points me to smth > I can read/fill missing points. > > Best Regards, > Elena. > > > > > > > > Now, suppose we were to convert i_count to refcount_t (yes, I know, my > > initial conversion wasn't well received), then we need to add > > futex_get_inode() similar to futex_get_mm(). > > > > That is, smp_mb__{before,after}_atomic() works as expected and can be > > used to fortify the implied barriers by refcount_t. > > > > --- > > Subject: fs,inode: Add comment explaining additional ordering > > > > Add a note to ihold() to document the ordering futex relies upon. > > > > Signed-off-by: Peter Zijlstra (Intel) > > --- > > fs/inode.c | 4 ++++ > > 1 file changed, 4 insertions(+) > > > > diff --git a/fs/inode.c b/fs/inode.c > > index 50370599e371..17192ba92fef 100644 > > --- a/fs/inode.c > > +++ b/fs/inode.c > > @@ -395,6 +395,10 @@ void __iget(struct inode *inode) > > */ > > void ihold(struct inode *inode) > > { > > + /* > > + * Note: futex.c:get_futex_key_refs() relies on this function > > + * implying an smp_mb(). > > + */ > > WARN_ON(atomic_inc_return(&inode->i_count) < 2); > > } > > EXPORT_SYMBOL(ihold);