linux-kernel - Re: [PATCH 09/12] xfs: refactor the ioend merging code

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Date:   Tue, 25 Jun 2019 15:42:20 +0300
From:   Nikolay Borisov <nborisov@...e.com>
To:     Christoph Hellwig <hch@....de>
Cc:     "Darrick J . Wong" <darrick.wong@...cle.com>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Andreas Gruenbacher <agruenba@...hat.com>,
        linux-xfs@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/12] xfs: refactor the ioend merging code



On 25.06.19 г. 13:14 ч., Christoph Hellwig wrote:
> On Mon, Jun 24, 2019 at 07:06:22PM +0300, Nikolay Borisov wrote:
>>> +{
>>> +	struct list_head	tmp;
>>> +
>>> +	list_replace_init(&ioend->io_list, &tmp);
>>> +	xfs_destroy_ioend(ioend, error);
>>> +	while ((ioend = list_pop(&tmp, struct xfs_ioend, io_list)))
>>> +		xfs_destroy_ioend(ioend, error);
>>
>> nit: I'd prefer if the list_pop patch is right before this one since
>> this is the first user of it.
> 
> I try to keep generic infrastructure first instead of interveawing
> it with subystem-specific patches.
> 
>> Additionally, I don't think list_pop is
>> really a net-negative win 
> 
> What is a "net-negative win" ?

What I meant was 'net-positive win', in terms of making the code more
readable or optimised.

> 
>> in comparison to list_for_each_entry_safe
>> here. In fact this "delete the list" would seems more idiomatic if
>> implemented via list_for_each_entry_safe
> 
> I disagree.  The for_each loops require an additional next iterator,
> and also don't clearly express what is going on, but require additional
> spotting of the list_del.

That is of course your opinion. At the very least we can agree to disagree.

What I'm worried about, though, is now you've essentially introduced a
new idiom to dispose of lists, which is used only in your code. If it
doesn't become more widespread and gradually start replacing current
list_for_each_entry_safe usage then you would have increased the public
list interface to cater for one specific use case, just because it seems
more natural to you. I guess only time will show whether it makes sense
to have list_pop_entry