Re: [PATCH 1/4] list: introduce list_is_first()

From: Jens Axboe
Date: Thu Dec 10 2015 - 10:35:16 EST

On 12/10/2015 08:23 AM, Josh Poimboeuf wrote:
On Thu, Dec 10, 2015 at 08:10:34AM -0700, Jens Axboe wrote:
On 12/10/2015 07:17 AM, Geliang Tang wrote:
We already have list_is_last(), it makes sense to also add
list_is_first() for consistency. This list utility function
to check for first element in a list.

Honestly, I think we already have way too many of these kind of helpers.
IMHO they don't really help, they hurt readability. You should know how the
list works anyway, and if you do, then it's a no-brainer what's first and
last. If you don't, then you are bound to screw up in other ways.

Just my 2 cents.

Personally I would disagree. Something like:

if (list_is_first(&rq->queuelist, &nd->queue))

is much more readable to me than:

if (rq->queuelist.prev == &nd->queue)

Both the function and your example are backwards, and hence a lot harder to comprehend than they should be. It'd be much clearer as:

if (nd-> == &rq->queuelist)

which is a lot easier to read. Nobody should open-code a 'is this the first entry in the list' by asking 'is the previous link to my node the head', asking 'is the next entry in the list X' makes a lot more sense. I'm assuming this happened because the list_is_last was just copied and modified, instead of thinking about this for a second.

The first one takes no effort for me -- it's almost English. While the
second one takes me a few seconds (and some precious brain cycles) to

Maybe whether it's readable depends on how many years you've been
looking at the pattern. But IMHO we shouldn't make "having x # of years
staring at kernel code" a prerequisite for being able to read kernel

It's a balance, as we also should not make APIs out of everything. As I said, purely my opinion, but I think the is_last/is_first have jumped the shark.

Jens Axboe

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