From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8CDA8C43381 for ; Thu, 14 Mar 2019 17:01:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 5448921855 for ; Thu, 14 Mar 2019 17:01:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727216AbfCNRB0 (ORCPT ); Thu, 14 Mar 2019 13:01:26 -0400 Received: from mx0.arrikto.com ([212.71.252.59]:34328 "EHLO mx0.arrikto.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726157AbfCNRB0 (ORCPT ); Thu, 14 Mar 2019 13:01:26 -0400 Received: from troi.prod.arr (mail.arr [10.99.0.5]) by mx0.arrikto.com (Postfix) with ESMTP id 3CF50182004; Thu, 14 Mar 2019 19:01:24 +0200 (EET) Received: from [10.89.50.3] (nivel.vpn.arr [10.89.50.3]) by troi.prod.arr (Postfix) with ESMTPSA id B4EB260; Thu, 14 Mar 2019 19:01:23 +0200 (EET) Subject: Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers To: paulmck@linux.ibm.com Cc: Mike Snitzer , linux-kernel@vger.kernel.org, iliastsi@arrikto.com, hch@infradead.org, dm-devel@redhat.com, mpatocka@redhat.com, agk@redhat.com References: <20181220180651.4879-1-ntsironis@arrikto.com> <20181220180651.4879-2-ntsironis@arrikto.com> <20190228213201.GB23527@redhat.com> <20190313234853.GA7797@linux.ibm.com> <20190314003027.GE4202@redhat.com> <20190314140750.GB4102@linux.ibm.com> From: Nikos Tsironis Message-ID: <86a26792-03ad-0dc4-737e-8fd5a3a79fa8@arrikto.com> Date: Thu, 14 Mar 2019 19:01:23 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.5.1 MIME-Version: 1.0 In-Reply-To: <20190314140750.GB4102@linux.ibm.com> Content-Type: text/plain; charset=windows-1252 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/14/19 4:07 PM, Paul E. McKenney wrote: > On Thu, Mar 14, 2019 at 03:28:23PM +0200, Nikos Tsironis wrote: >> On 3/14/19 2:30 AM, Mike Snitzer wrote: >>> On Wed, Mar 13 2019 at 7:48pm -0400, >>> Paul E. McKenney wrote: >>> >> Hi Paul, >> >> Thanks a lot for your feedback! > > NP, and apologies for the delay. > >>>> On Thu, Feb 28, 2019 at 04:32:02PM -0500, Mike Snitzer wrote: >>>>> On Thu, Dec 20 2018 at 1:06pm -0500, >>>>> Nikos Tsironis wrote: >>>>> >>>>>> Add hlist_bl_add_before/behind helpers to add an element before/after an >>>>>> existing element in a bl_list. >>>>>> >>>>>> Signed-off-by: Nikos Tsironis >>>>>> Signed-off-by: Ilias Tsitsimpis >>>>>> --- >>>>>> include/linux/list_bl.h | 27 +++++++++++++++++++++++++++ >>>>>> 1 file changed, 27 insertions(+) >>>>>> >>>>>> diff --git a/include/linux/list_bl.h b/include/linux/list_bl.h >>>>>> index 3fc2cc57ba1b..2fd918e5fd48 100644 >>>>>> --- a/include/linux/list_bl.h >>>>>> +++ b/include/linux/list_bl.h >>>>>> @@ -86,6 +86,33 @@ static inline void hlist_bl_add_head(struct hlist_bl_node *n, >>>>>> hlist_bl_set_first(h, n); >>>>>> } >>>>>> >>>>>> +static inline void hlist_bl_add_before(struct hlist_bl_node *n, >>>>>> + struct hlist_bl_node *next) >>>>>> +{ >>>>>> + struct hlist_bl_node **pprev = next->pprev; >>>>>> + >>>>>> + n->pprev = pprev; >>>>>> + n->next = next; >>>>>> + next->pprev = &n->next; >>>>>> + >>>>>> + /* pprev may be `first`, so be careful not to lose the lock bit */ >>>>>> + WRITE_ONCE(*pprev, >>>>>> + (struct hlist_bl_node *) >>>>>> + ((unsigned long)n | >>>>>> + ((unsigned long)*pprev & LIST_BL_LOCKMASK))); >>>> >>>> A nit, but use of uintptr_t shrinks things a bit: >>>> >>>> + (struct hlist_bl_node *) >>>> + ((uintptr_t)n | ((uintptr_t)*pprev & LIST_BL_LOCKMASK))); >>>> >>>> I am not too concerned about this, though. >>> >>> I'm fine with folding in your suggestion. >> >> Indeed, this looks better. >> >>>> The WRITE_ONCE() is to handle races with hlist_bl_empty() (which does contain >>>> the corresponding READ_ONCE()) correct? >>> >>> Correct. >> >> Yes that's correct. >> >>>>>> +} >>>>>> + >>>>>> +static inline void hlist_bl_add_behind(struct hlist_bl_node *n, >>>>>> + struct hlist_bl_node *prev) >>>>>> +{ >>>>>> + n->next = prev->next; >>>>>> + n->pprev = &prev->next; >>>>>> + WRITE_ONCE(prev->next, n); >>>> >>>> I don't see what this WRITE_ONCE() is interacting with. The traversals >>>> use plain C-language reads, and hlist_bl_empty() can't get here. All >>>> uses of hlist_bl_for_each_entry() invoke hlist_bl_lock() before starting >>>> the traversal, and hlist_bl_for_each_entry_safe() looks to be unused. >>>> (Perhaps it should be removed? Or is there some anticipated use?) >> >> I am using hlist_bl_for_each_entry_safe() in this proposed patch for >> dm-snapshot: https://patchwork.kernel.org/patch/10835709/ > > Probably should keep it, then. ;-) > >>>> >>>> I don't believe that the WRITE_ONCE() is needed. What am I missing? >>>> >>>> Other than that, looks good. >>>> >>>> Thanx, Paul >>>> >>> >>> I'd imagine it was just born out of symmetry with hlist_bl_add_before() >>> and/or caution. But let's see what Nikos has to say. >> >> I also don't believe that this WRITE_SAME() is needed. But, looking at >> hlist_add_behind() in include/linux/list.h, which, if I am not missing >> something, is used in the same way as hlist_bl_add_behind(), it also >> uses WRITE_ONCE() to update prev->next: >> >> static inline void hlist_add_behind(struct hlist_node *n, >> struct hlist_node *prev) >> { >> n->next = prev->next; >> WRITE_ONCE(prev->next, n); >> n->pprev = &prev->next; >> >> if (n->next) >> n->next->pprev = &n->next; >> } >> >> Could it be the case that the WRITE_ONCE() in hlist_add_behind() is also >> not needed? This WRITE_ONCE() was introduced by commit 1c97be677f72b3 >> ("list: Use WRITE_ONCE() when adding to lists and hlists"). > > Looks like I have no one to blame but myself! > > Would you like to remove that as part of your patch series? Yes, Of course. I will add an extra patch removing the WRITE_ONCE() from hlist_add_behind(). Thanks, Nikos > >> But, since I am not an expert in lockless programming, I opted to be on >> the safe side and followed the example of hlist_add_behind(). >> >> That said, I will follow up with a new version of the patch removing the >> WRITE_ONCE() and using uintptr_t instead of unsigned long. > > Sounds good! > > Thanx, Paul > > -- > dm-devel mailing list > dm-devel@redhat.com > https://www.redhat.com/mailman/listinfo/dm-devel >