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,URIBL_BLOCKED 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 00A09C43381 for ; Thu, 14 Mar 2019 13:35:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id CDE0B2085A for ; Thu, 14 Mar 2019 13:35:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727518AbfCNNfl (ORCPT ); Thu, 14 Mar 2019 09:35:41 -0400 Received: from mx0.arrikto.com ([212.71.252.59]:33699 "EHLO mx0.arrikto.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726845AbfCNNfk (ORCPT ); Thu, 14 Mar 2019 09:35:40 -0400 X-Greylist: delayed 434 seconds by postgrey-1.27 at vger.kernel.org; Thu, 14 Mar 2019 09:35:38 EDT Received: from troi.prod.arr (mail.arr [10.99.0.5]) by mx0.arrikto.com (Postfix) with ESMTP id E91E3182004; Thu, 14 Mar 2019 15:28:23 +0200 (EET) Received: from [10.94.250.119] (dhcp-119.hq.arr [10.94.250.119]) by troi.prod.arr (Postfix) with ESMTPSA id 924EF60; Thu, 14 Mar 2019 15:28:23 +0200 (EET) Subject: Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers To: Mike Snitzer , "Paul E. McKenney" Cc: hch@infradead.org, agk@redhat.com, dm-devel@redhat.com, mpatocka@redhat.com, iliastsi@arrikto.com, linux-kernel@vger.kernel.org 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> From: Nikos Tsironis Message-ID: Date: Thu, 14 Mar 2019 15:28:23 +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: <20190314003027.GE4202@redhat.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 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! >> 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/ >> >> 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"). 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. Thanks, Nikos