Re: [PATCH v2 01/26] rculist: introduce list_next_or_null_rr_rcu()

[Roman Penyaev <roman.penyaev@profitbricks.com>]

On Sat, May 19, 2018 at 6:37 PM, Paul E. McKenney
<paulmck@linux.vnet.ibm.com> wrote:
> On Fri, May 18, 2018 at 03:03:48PM +0200, Roman Pen wrote:
>> Function is going to be used in transport over RDMA module
>> in subsequent patches.
>>
>> Function returns next element in round-robin fashion,
>> i.e. head will be skipped.  NULL will be returned if list
>> is observed as empty.
>>
>> Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
>> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
>> Cc: linux-kernel@vger.kernel.org
>> ---
>>  include/linux/rculist.h | 19 +++++++++++++++++++
>>  1 file changed, 19 insertions(+)
>>
>> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
>> index 127f534fec94..b0840d5ab25a 100644
>> --- a/include/linux/rculist.h
>> +++ b/include/linux/rculist.h
>> @@ -339,6 +339,25 @@ static inline void list_splice_tail_init_rcu(struct list_head *list,
>>  })
>>
>>  /**
>> + * list_next_or_null_rr_rcu - get next list element in round-robin fashion.
>> + * @head:    the head for the list.
>> + * @ptr:        the list head to take the next element from.
>> + * @type:       the type of the struct this is embedded in.
>> + * @memb:       the name of the list_head within the struct.
>> + *
>> + * Next element returned in round-robin fashion, i.e. head will be skipped,
>> + * but if list is observed as empty, NULL will be returned.
>> + *
>> + * This primitive may safely run concurrently with the _rcu list-mutation
>> + * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
>
> Of course, all the set of list_next_or_null_rr_rcu() invocations that
> are round-robining a given list must all be under the same RCU read-side
> critical section.  For example, the following will break badly:
>
>         struct foo *take_rr_step(struct list_head *head, struct foo *ptr)
>         {
>                 struct foo *ret;
>
>                 rcu_read_lock();
>                 ret = list_next_or_null_rr_rcu(head, ptr, struct foo, foolist);
>                 rcu_read_unlock();  /* BUG */
>                 return ret;
>         }
>
> You need a big fat comment stating this, at the very least.  The resulting
> bug can be very hard to trigger and even harder to debug.
>
> And yes, I know that the same restriction applies to list_next_rcu()
> and friends.  The difference is that if you try to invoke those in an
> infinite loop, you will be rapped on the knuckles as soon as you hit
> the list header.  Without that knuckle-rapping, RCU CPU stall warnings
> might tempt people to do something broken like take_rr_step() above.

Hi Paul,

I need -rr behaviour for doing IO load-balancing when I choose next RDMA
connection from the list in order to send a request, i.e. my code is
something like the following:

        static struct conn *get_and_set_next_conn(void)
        {
                struct conn *conn;

                conn = rcu_dereferece(rcu_conn);
                if (unlikely(!conn))
                    return conn;
                conn = list_next_or_null_rr_rcu(&conn_list,
                                                &conn->entry,
                                                typeof(*conn),
                                                entry);
                rcu_assign_pointer(rcu_conn, conn);
                return conn;
        }

        rcu_read_lock();
        conn = get_and_set_next_conn();
        if (unlikely(!conn)) {
                /* ... */
        }
        err = rdma_io(conn, request);
        rcu_read_unlock();

i.e. usage of the @next pointer is under an RCU critical section.

> Is it possible to instead do some sort of list_for_each_entry_rcu()-like
> macro that makes it more obvious that the whole thing need to be under
> a single RCU read-side critical section?  Such a macro would of course be
> an infinite loop if the list never went empty, so presumably there would
> be a break or return statement in there somewhere.

The difference is that I do not need a loop, I take the @next conn pointer,
save it for the following IO request and do IO for current IO request.

It seems list_for_each_entry_rcu()-like with immediate "break" in the body
of the loop does not look nice, I personally do not like it, i.e.:


        static struct conn *get_and_set_next_conn(void)
        {
                struct conn *conn;

                conn = rcu_dereferece(rcu_conn);
                if (unlikely(!conn))
                    return conn;
                list_for_each_entry_rr_rcu(conn, &conn_list,
                                           entry) {
                        break;
                }
                rcu_assign_pointer(rcu_conn, conn);
                return conn;
        }


or maybe I did not fully get your idea?

>> + */
>> +#define list_next_or_null_rr_rcu(head, ptr, type, memb) \
>> +({ \
>> +     list_next_or_null_rcu(head, ptr, type, memb) ?: \
>> +             list_next_or_null_rcu(head, READ_ONCE((ptr)->next), type, memb); \
>
> Are there any uses for this outside of RDMA?  If not, I am with Linus.
> Define this within RDMA, where a smaller number of people can more
> easily be kept aware of the restrictions on use.  If it turns out to be
> more generally useful, we can take a look at exactly what makes sense
> more globally.

The only one list_for_each_entry_rcu()-like macro I am aware of is used in
block/blk-mq-sched.c, is called list_for_each_entry_rcu_rr():

https://elixir.bootlin.com/linux/v4.17-rc5/source/block/blk-mq-sched.c#L370

Does it make sense to implement generic list_next_or_null_rr_rcu() reusing
my list_next_or_null_rr_rcu() variant?

> Even within RDMA, I strongly recommend the big fat comment called out above.
> And the list_for_each_entry_rcu()-like formulation, if that can be made to
> work within RDMA's code structure.
>
> Seem reasonable, or am I missing something here?

Thanks for clear explanation.

--
Roman