Re: [PATCH net-next 02/15] net: dsa: sja1105: Remove usage of iterator for list_add() after loop

[Jakob Koschel <jakobkoschel@gmail.com>]

Hey Christophe,

> On 8. Apr 2022, at 09:47, Christophe Leroy <christophe.leroy@csgroup.eu> wrote:
> 
> 
> 
> Le 07/04/2022 à 12:28, Jakob Koschel a écrit :
>> In preparation to limit the scope of a list iterator to the list
>> traversal loop, use a dedicated pointer to point to the found element [1].
>> 
>> Before, the code implicitly used the head when no element was found
>> when using &pos->list. Since the new variable is only set if an
>> element was found, the list_add() is performed within the loop
>> and only done after the loop if it is done on the list head directly.
>> 
>> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
>> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
>> ---
>> drivers/net/dsa/sja1105/sja1105_vl.c | 14 +++++++++-----
>> 1 file changed, 9 insertions(+), 5 deletions(-)
>> 
>> diff --git a/drivers/net/dsa/sja1105/sja1105_vl.c b/drivers/net/dsa/sja1105/sja1105_vl.c
>> index b7e95d60a6e4..cfcae4d19eef 100644
>> --- a/drivers/net/dsa/sja1105/sja1105_vl.c
>> +++ b/drivers/net/dsa/sja1105/sja1105_vl.c
>> @@ -27,20 +27,24 @@ static int sja1105_insert_gate_entry(struct sja1105_gating_config *gating_cfg,
>> 	if (list_empty(&gating_cfg->entries)) {
>> 		list_add(&e->list, &gating_cfg->entries);
>> 	} else {
>> -		struct sja1105_gate_entry *p;
>> +		struct sja1105_gate_entry *p = NULL, *iter;
>> 
>> -		list_for_each_entry(p, &gating_cfg->entries, list) {
>> -			if (p->interval == e->interval) {
>> +		list_for_each_entry(iter, &gating_cfg->entries, list) {
>> +			if (iter->interval == e->interval) {
>> 				NL_SET_ERR_MSG_MOD(extack,
>> 						 "Gate conflict");
>> 				rc = -EBUSY;
>> 				goto err;
>> 			}
>> 
>> -			if (e->interval < p->interval)
>> +			if (e->interval < iter->interval) {
>> +				p = iter;
>> +				list_add(&e->list, iter->list.prev);
>> 				break;
>> +			}
>> 		}
>> -		list_add(&e->list, p->list.prev);
>> +		if (!p)
>> +			list_add(&e->list, gating_cfg->entries.prev);
>> 	}
>> 
>> 	gating_cfg->num_entries++;
> 
> This change looks ugly, why duplicating the list_add() to do the same ? 
> At the end of the loop the pointer contains gating_cfg->entries, so it 
> was cleaner before.
> 
> If you don't want to use the loop index outside the loop, fair enough, 
> all you have to do is:
> 
> 		struct sja1105_gate_entry *p, *iter;
> 
> 		list_for_each_entry(iter, &gating_cfg->entries, list) {
> 			if (iter->interval == e->interval) {
> 				NL_SET_ERR_MSG_MOD(extack,
> 						 "Gate conflict");
> 				rc = -EBUSY;
> 				goto err;
> 			}
> 			p = iter;
> 
> 			if (e->interval < iter->interval)
> 				break;
> 		}
> 		list_add(&e->list, p->list.prev);

Thanks for the review and input.

The code you are showing here would have an uninitialized access to 'p'
if the list is empty.

Also 'p->list.prev' will be the second last entry if the list iterator
ran through completely, whereas the original code was pointing to the last
entry of the list.

IMO Vladimir Oltean posted a nice alternative way to solve it, see [1].
That way it keeps the semantics of the code the same and doesn't duplicate
the list_add.

> 
> 
> 
> Christophe

[1] https://lore.kernel.org/linux-kernel/20220408114120.tvf2lxvhfqbnrlml@skbuf/

Thanks,
Jakob