linux-kernel - Re: [PATCH v5 03/78] xarray: Add the xa_lock to the radix_tree

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Date:   Tue, 2 Jan 2018 14:41:37 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     "Darrick J. Wong" <darrick.wong@...cle.com>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        linux-kernel@...r.kernel.org,
        Matthew Wilcox <mawilcox@...rosoft.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        David Howells <dhowells@...hat.com>,
        Shaohua Li <shli@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Rehas Sachdeva <aquannie@...il.com>,
        Marc Zyngier <marc.zyngier@....com>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-nilfs@...r.kernel.org, linux-btrfs@...r.kernel.org,
        linux-xfs@...r.kernel.org, linux-usb@...r.kernel.org,
        linux-raid@...r.kernel.org
Subject: Re: [PATCH v5 03/78] xarray: Add the xa_lock to the radix_tree_root

On Tue, Jan 02, 2018 at 10:01:55AM -0800, Darrick J. Wong wrote:
> On Tue, Dec 26, 2017 at 07:58:15PM -0800, Matthew Wilcox wrote:
> >         spin_lock_irqsave(&mapping->pages, flags);
> >         __delete_from_page_cache(page, NULL);
> >         spin_unlock_irqrestore(&mapping->pages, flags);
> >
> > More details here: https://9p.io/sys/doc/compiler.html
> 
> I read the link, and I understand (from section 3.3) that replacing
> foo.bar.baz.goo with foo.goo is less typing, but otoh the first time I
> read your example above I thought "we're passing (an array of pages |
> something that doesn't have the word 'lock' in the name) to
> spin_lock_irqsave? wtf?"

I can see that being a bit jarring initially.  If you think about what
object-oriented languages were offering in the nineties, this is basically
C++ multiple-inheritance / Java interfaces.  So when I read the above
example, I think "lock the mapping pages, delete from page cache, unlock
the mapping pages", and I don't have a wtf moment.  It's just simpler to
read than "lock the mapping pages lock", and less redundant.

> I suppose it does force me to go dig into whatever mapping->pages is to
> figure out that there's an unnamed spinlock_t and that the compiler can
> insert the appropriate pointer arithmetic, but now my brain trips over
> 'pages' being at the end of the selector for parameter 1 which slows
> down my review reading...
> 
> OTOH I guess it /did/ motivate me to click the link, so well played,
> sir. :)

Now if only I can trick you into giving your ACK on patch 1,
"xfs: Rename xa_ elements to ail_"