Re: [PATCH net-next 11/15] sfc: Remove usage of list iterator for list_add() after the loop body

[Jakob Koschel <jakobkoschel@gmail.com>]

Hello Edward,

> On 7. Apr 2022, at 19:42, Edward Cree <ecree.xilinx@gmail.com> wrote:
> 
> On 07/04/2022 11:28, Jakob Koschel wrote:
>> 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>
> 
> The commit message doesn't accurately describe the patch; it states
> that "the list_add() is performed within the loop", which doesn't
> appear to be the case.

you're right, I've changed the code last minute. I'll make sure
the changelog reflects the actual behaviour here. Thanks for the
input!

> Also it seems a bit subtle to use `head` as both the head of the
> list to iterate over and the found entry/gap to insert before; a
> comment explaining that wouldn't go amiss.

Also a good point, I'll add a comment as well, or perhaps using
a separate 'struct list_head *pos' variable is even cleaner.

> (I'd question whether this change is really an improvement in this
> case, where the iterator really does hold the thing we want at the
> end of the search and so there's no if(found) special-casing —
> we're not even abusing the type system, because efx->rss_context
> is of the same type as all the list entries, so ctx really is a
> valid pointer and there shouldn't be any issues with speculative
> accesses or whatever — but it seems Linus has already pronounced
> in favour of the scope limiting, and far be it from me to gainsay
> him.)

So, since the head is included in the struct of the same type as
the element, it really doesn't make much of a difference here.
It will always be safe to use.

But this is the very rare exception. There are other benefits of
avoiding the use of list iterator after the loop. One of them
is scope limiting but you also might want to set the iterator
variable to a "safe value" before the processor might execute an additional
iteration in speculative execution on the 'bogus' head element.

If you do these kind of patches on the list macros, you need to make sure
they work for all the uses, including the safe ones (like this one).

> 
> -ed
> 
>> ---
>> drivers/net/ethernet/sfc/rx_common.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>> 
>> diff --git a/drivers/net/ethernet/sfc/rx_common.c b/drivers/net/ethernet/sfc/rx_common.c
>> index 1b22c7be0088..a8822152ff83 100644
>> --- a/drivers/net/ethernet/sfc/rx_common.c
>> +++ b/drivers/net/ethernet/sfc/rx_common.c
>> @@ -563,8 +563,10 @@ struct efx_rss_context *efx_alloc_rss_context_entry(struct efx_nic *efx)
>> 
>> 	/* Search for first gap in the numbering */
>> 	list_for_each_entry(ctx, head, list) {
>> -		if (ctx->user_id != id)
>> +		if (ctx->user_id != id) {
>> +			head = &ctx->list;
>> 			break;
>> +		}
>> 		id++;
>> 		/* Check for wrap.  If this happens, we have nearly 2^32
>> 		 * allocated RSS contexts, which seems unlikely.
>> @@ -582,7 +584,7 @@ struct efx_rss_context *efx_alloc_rss_context_entry(struct efx_nic *efx)
>> 
>> 	/* Insert the new entry into the gap */
>> 	new->user_id = id;
>> -	list_add_tail(&new->list, &ctx->list);
>> +	list_add_tail(&new->list, head);
>> 	return new;
>> }
>> 
>> 
> 

Thanks,
Jakob