Date:   Wed, 19 Aug 2020 07:17:19 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     James Bottomley <James.Bottomley@...senpartnership.com>,
        Kees Cook <keescook@...omium.org>, ulf.hansson@...aro.org,
        linux-atm-general@...ts.sourceforge.net, manohar.vanga@...il.com,
        airlied@...ux.ie, Allen Pais <allen.lkml@...il.com>,
        linux-hyperv@...r.kernel.org, dri-devel@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, anton.ivanov@...bridgegreys.com,
        devel@...verdev.osuosl.org, linux-s390@...r.kernel.org,
        linux1394-devel@...ts.sourceforge.net, maximlevitsky@...il.com,
        richard@....at, deller@....de, jassisinghbrar@...il.com,
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        Allen Pais <allen.cryptic@...il.com>,
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        Romain Perier <romain.perier@...il.com>, shawnguo@...nel.org,
        davem@...emloft.net
Subject: Re: [PATCH] block: convert tasklets to use new tasklet_setup() API

On 8/19/20 6:11 AM, Greg KH wrote:
> On Wed, Aug 19, 2020 at 07:00:53AM -0600, Jens Axboe wrote:
>> On 8/18/20 1:00 PM, James Bottomley wrote:
>>> On Mon, 2020-08-17 at 13:02 -0700, Jens Axboe wrote:
>>>> On 8/17/20 12:48 PM, Kees Cook wrote:
>>>>> On Mon, Aug 17, 2020 at 12:44:34PM -0700, Jens Axboe wrote:
>>>>>> On 8/17/20 12:29 PM, Kees Cook wrote:
>>>>>>> On Mon, Aug 17, 2020 at 06:56:47AM -0700, Jens Axboe wrote:
>>>>>>>> On 8/17/20 2:15 AM, Allen Pais wrote:
>>>>>>>>> From: Allen Pais <allen.lkml@...il.com>
>>>>>>>>>
>>>>>>>>> In preparation for unconditionally passing the
>>>>>>>>> struct tasklet_struct pointer to all tasklet
>>>>>>>>> callbacks, switch to using the new tasklet_setup()
>>>>>>>>> and from_tasklet() to pass the tasklet pointer explicitly.
>>>>>>>>
>>>>>>>> Who came up with the idea to add a macro 'from_tasklet' that
>>>>>>>> is just container_of? container_of in the code would be
>>>>>>>> _much_ more readable, and not leave anyone guessing wtf
>>>>>>>> from_tasklet is doing.
>>>>>>>>
>>>>>>>> I'd fix that up now before everything else goes in...
>>>>>>>
>>>>>>> As I mentioned in the other thread, I think this makes things
>>>>>>> much more readable. It's the same thing that the timer_struct
>>>>>>> conversion did (added a container_of wrapper) to avoid the
>>>>>>> ever-repeating use of typeof(), long lines, etc.
>>>>>>
>>>>>> But then it should use a generic name, instead of each sub-system 
>>>>>> using some random name that makes people look up exactly what it
>>>>>> does. I'm not huge fan of the container_of() redundancy, but
>>>>>> adding private variants of this doesn't seem like the best way
>>>>>> forward. Let's have a generic helper that does this, and use it
>>>>>> everywhere.
>>>>>
>>>>> I'm open to suggestions, but as things stand, these kinds of
>>>>> treewide
>>>>
>>>> On naming? Implementation is just as it stands, from_tasklet() is
>>>> totally generic which is why I objected to it. from_member()? Not
>>>> great with naming... But I can see this going further and then we'll
>>>> suddenly have tons of these. It's not good for readability.
>>>
>>> Since both threads seem to have petered out, let me suggest in
>>> kernel.h:
>>>
>>> #define cast_out(ptr, container, member) \
>>> 	container_of(ptr, typeof(*container), member)
>>>
>>> It does what you want, the argument order is the same as container_of
>>> with the only difference being you name the containing structure
>>> instead of having to specify its type.
>>
>> Not to incessantly bike shed on the naming, but I don't like cast_out,
>> it's not very descriptive. And it has connotations of getting rid of
>> something, which isn't really true.
> 
> I agree, if we want to bike shed, I don't like this color either.
> 
>> FWIW, I like the from_ part of the original naming, as it has some clues
>> as to what is being done here. Why not just from_container()? That
>> should immediately tell people what it does without having to look up
>> the implementation, even before this becomes a part of the accepted
>> coding norm.
> 
> Why are people hating on the well-known and used container_of()?
> 
> If you really hate to type the type and want a new macro, what about
> 'container_from()'?  (noun/verb is nicer to sort symbols by...)
> 
> But really, why is this even needed?

container_from() or from_container(), either works just fine for me
in terms of naming.

I think people are hating on it because it makes for _really_ long
lines, and it's arguably cleaner/simpler to just pass in the pointer
type instead. Then you end up with lines like this:

	struct request_queue *q =                                               
		container_of(work, struct request_queue, requeue_work.work);  

But I'm not the one that started this addition of from_tasklet(), my
objection was adding a private macro for something that should be
generic functionality. Hence I think we either need to provide that, or
tell the from_tasklet() folks that they should just use container_of().

-- 
Jens Axboe

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