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McKenney" To: Byungchul Park Cc: linux-kernel@vger.kernel.org, kernel-team@lge.com, ying.huang@intel.com, peterz@infradead.org, mingo@kernel.org, jiangshanlai@gmail.com, josh@joshtriplett.org, rostedt@goodmis.org, mathieu.desnoyers@efficios.com, joel@joelfernandes.org, len.brown@intel.com, glider@google.com, peter@hurleysoftware.com, aik@ozlabs.ru Subject: Re: [QUESTION] llist: Comment releasing 'must delete' restriction before traversing Reply-To: paulmck@linux.vnet.ibm.com References: <1532998716-5037-1-git-send-email-byungchul.park@lge.com> <20180731043042.GJ24813@linux.vnet.ibm.com> <20180731092950.GB12241@X58A-UD3R> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180731092950.GB12241@X58A-UD3R> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18073114-0072-0000-0000-0000038821E0 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009463; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01068479; UDB=6.00549296; IPR=6.00846676; MB=3.00022427; MTD=3.00000008; XFM=3.00000015; UTC=2018-07-31 14:30:56 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18073114-0073-0000-0000-000048E8353E Message-Id: <20180731143052.GL24813@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-07-31_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=8.0.1-1806210000 definitions=main-1807310137 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jul 31, 2018 at 06:29:50PM +0900, Byungchul Park wrote: > On Mon, Jul 30, 2018 at 09:30:42PM -0700, Paul E. McKenney wrote: > > On Tue, Jul 31, 2018 at 09:58:36AM +0900, Byungchul Park wrote: > > > Hello folks, > > > > > > I'm careful in saying.. and curious about.. > > > > > > In restrictive cases like only addtions happen but never deletion, can't > > > we safely traverse a llist? I believe llist can be more useful if we can > > > release the restriction. Can't we? > > > > Yes, but please give a thought to the people looking at your code some > > time down the line. If you are doing this, lots of comments, please. > > Yes, I will. Thank you for the comment. > > > Here are the approaches that I am aware of: > > > > 1. Normal RCU. Use list_add_rcu(), list_del_rcu(), and friends. > > > > 2. Things are added but never deleted. Use list_add_rcu() and > > friends, but since you don't ever delete anything, you never > > use list_del_rcu(), synchronize_rcu(), call_rcu(), and friends. > > I think rcu list also works well. But I decided to use llist because > llist is simpler and has one less pointer. No. To see this, look at llist_for_each() below, which is absolutely -not- able to reliably traverse lists while nodes are being inserted. #define llist_for_each(pos, node) \ for ((pos) = (node); pos; (pos) = (pos)->next) Now, you could introduce an llist_for_each_rcu() that used rcu_dereference or similar (thus handling insertion, but that is not what your patches currently do. > Just to be sure, let me explain my use case more: > > 1. Introduced a global list where single linked list is sufficient. > 2. All operations I need is to add items and traverse the list. > 3. Ensure the operations always happen within irq-disabled section. > 4. I'm considering CONFIG_ARCH_HAVE_NMI_SAFE_CMPXCHG properly. > 5. The list can be accessed by every CPU concurrently. > > Of course, I can use normal double list with a lock or rcu list. But I > think it doesn't have to be protected by even rcu in that case. I wanted > to use the simplest one all requiremnets are satisfied with and I > thought it's llist. Thoughts? If you want lockless reader traversal, you need rcu_dereference(). > > 5. Just mark the deleted elements, but leave them in the list. > > Actually remove them using one of the above techniques. > > Honestly, I have a plan to do this thing as a future work. But now, I > can assume deletion never happen with the list :) > > > I suggest that such special cases stay in the subsystem in question. > > If a given technique gains wider use, then it might be time to > > update header comments. > > Ok. > > > > If yes, we may add another function traversing starting from a head. Or > > > just use existing funtion with head->first. > > > > If you start with head->first, then you need to make sure that a concurrent > > add of an element at the head of the list works. You need at least a > > READ_ONCE() and preferably an rcu_dereference() or similar. > > Yes, sir. I'll be careful in doing it. Which means adding something to the current llist.h. Thanx, Paul > Thanks a lot. > > > > Thank a lot for your answers in advance :) > > > > You did ask! > > > > Thanx, Paul > > > > > ----->8----- > > > >From 1e73882799b269cd86e7a7c955021e3a18d1e6cf Mon Sep 17 00:00:00 2001 > > > From: Byungchul Park > > > Date: Tue, 31 Jul 2018 09:31:57 +0900 > > > Subject: [QUESTION] llist: Comment releasing 'must delete' restriction before > > > traversing > > > > > > llist traversing can run without deletion in restrictive cases all > > > items are added but never deleted like a rculist traversing such as > > > list_for_each_entry_lockless. So add the comment. > > > > > > Signed-off-by: Byungchul Park > > > --- > > > include/linux/llist.h | 24 ++++++++++++++++++------ > > > 1 file changed, 18 insertions(+), 6 deletions(-) > > > > > > diff --git a/include/linux/llist.h b/include/linux/llist.h > > > index 85abc29..d012d3e 100644 > > > --- a/include/linux/llist.h > > > +++ b/include/linux/llist.h > > > @@ -32,8 +32,12 @@ > > > * operation, with "-" being no lock needed, while "L" being lock is needed. > > > * > > > * The list entries deleted via llist_del_all can be traversed with > > > - * traversing function such as llist_for_each etc. But the list > > > - * entries can not be traversed safely before deleted from the list. > > > + * traversing function such as llist_for_each etc. Normally the list > > > + * entries cannot be traversed safely before deleted from the list > > > + * except the cases items are added to the list but never deleted. In > > > + * that restrictive cases the list may be safely traversed concurrently > > > + * with llist_add. > > > + * > > > * The order of deleted entries is from the newest to the oldest added > > > * one. If you want to traverse from the oldest to the newest, you > > > * must reverse the order by yourself before traversing. > > > @@ -116,7 +120,9 @@ static inline void init_llist_head(struct llist_head *list) > > > * > > > * In general, some entries of the lock-less list can be traversed > > > * safely only after being deleted from list, so start with an entry > > > - * instead of list head. > > > + * instead of list head. But in restrictive cases items are added to > > > + * the list but never deleted, the list may be safely traversed > > > + * concurrently with llist_add. > > > * > > > * If being used on entries deleted from lock-less list directly, the > > > * traverse order is from the newest to the oldest added entry. If > > > @@ -135,7 +141,9 @@ static inline void init_llist_head(struct llist_head *list) > > > * > > > * In general, some entries of the lock-less list can be traversed > > > * safely only after being deleted from list, so start with an entry > > > - * instead of list head. > > > + * instead of list head. But in restrictive cases items are added to > > > + * the list but never deleted, the list may be safely traversed > > > + * concurrently with llist_add. > > > * > > > * If being used on entries deleted from lock-less list directly, the > > > * traverse order is from the newest to the oldest added entry. If > > > @@ -153,7 +161,9 @@ static inline void init_llist_head(struct llist_head *list) > > > * > > > * In general, some entries of the lock-less list can be traversed > > > * safely only after being removed from list, so start with an entry > > > - * instead of list head. > > > + * instead of list head. But in restrictive cases items are added to > > > + * the list but never deleted, the list may be safely traversed > > > + * concurrently with llist_add. > > > * > > > * If being used on entries deleted from lock-less list directly, the > > > * traverse order is from the newest to the oldest added entry. If > > > @@ -175,7 +185,9 @@ static inline void init_llist_head(struct llist_head *list) > > > * > > > * In general, some entries of the lock-less list can be traversed > > > * safely only after being removed from list, so start with an entry > > > - * instead of list head. > > > + * instead of list head. But in restrictive cases items are added to > > > + * the list but never deleted, the list may be safely traversed > > > + * concurrently with llist_add. > > > * > > > * If being used on entries deleted from lock-less list directly, the > > > * traverse order is from the newest to the oldest added entry. If > > > -- > > > 1.9.1 > > > >