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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> 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: 19031414-0060-0000-0000-0000031C5B50 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010757; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000281; SDB=6.01174264; UDB=6.00614020; IPR=6.00954929; MB=3.00025978; MTD=3.00000008; XFM=3.00000015; UTC=2019-03-14 14:07:08 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19031414-0061-0000-0000-0000489B8D96 Message-Id: <20190314140750.GB4102@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-03-14_06:,, 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-1903140099 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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! Thanx, Paul