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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Mon, 18 Mar 2019 17:15:52 -0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23032.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x2IHFpff23330850 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 18 Mar 2019 17:15:52 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id E27ABB2064; Mon, 18 Mar 2019 17:15:51 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B4F62B205F; Mon, 18 Mar 2019 17:15:51 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.188]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 18 Mar 2019 17:15:51 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 602E116C0843; Mon, 18 Mar 2019 10:16:40 -0700 (PDT) Date: Mon, 18 Mar 2019 10:16:40 -0700 From: "Paul E. McKenney" To: Nikos Tsironis Cc: Mike Snitzer , hch@infradead.org, agk@redhat.com, dm-devel@redhat.com, mpatocka@redhat.com, iliastsi@arrikto.com, linux-kernel@vger.kernel.org Subject: Re: [dm-devel] [PATCH 1/3] list_bl: Add hlist_bl_add_before/behind helpers Reply-To: paulmck@linux.ibm.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> <20190314150306.GA22051@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19031817-0064-0000-0000-000003BBEF7D X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010780; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000281; SDB=6.01176214; UDB=6.00615201; IPR=6.00956898; MB=3.00026036; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-18 17:15:53 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19031817-0065-0000-0000-00003CC2EC4F Message-Id: <20190318171640.GM4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-18_11:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1903180127 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Mar 17, 2019 at 01:52:50PM +0200, Nikos Tsironis wrote: > On 3/14/19 5:03 PM, Paul E. McKenney wrote: > > On Thu, Mar 14, 2019 at 07:07:50AM -0700, 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? > >> > >>> 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! > > > > Oh, and of course intptr_t is one character shorter than uintptr_t, and > > looks to work just as well in this context. ;-) > > > > Thanx, Paul > > > > > Hi Paul, > > Sorry for the late reply. > > intptr_t seems to be defined only in a header file under arch/mips, so I > will stick to uintptr_t. Ah, apologies for the misdirection! Hmmm... Maybe intptr_t should be added alongside uintptr_t? Saving a character is saving a character. ;-) Thanx, Paul