Re: [PATCH v5 03/78] xarray: Add the xa_lock to the radix_tree_root

[Darrick J. Wong]

On Tue, Dec 26, 2017 at 07:58:15PM -0800, Matthew Wilcox wrote:
> On Tue, Dec 26, 2017 at 07:43:40PM -0800, Matthew Wilcox wrote:
> >     Also add the xa_lock() and xa_unlock() family of wrappers to make it
> >     easier to use the lock.  If we could rely on -fplan9-extensions in
> >     the compiler, we could avoid all of this syntactic sugar, but that
> >     wasn't added until gcc 4.6.
> 
> Oh, in case anyone's wondering, here's how I'd do it with plan9 extensions:
> 
> struct xarray {
>         spinlock_t;
>         int xa_flags;
>         void *xa_head;
> };
> 
> ...
>         spin_lock_irqsave(&mapping->pages, flags);
>         __delete_from_page_cache(page, NULL);
>         spin_unlock_irqrestore(&mapping->pages, flags);
> ...
> 
> The plan9 extensions permit passing a pointer to a struct which has an
> unnamed element to a function which is expecting a pointer to the type
> of that element.  The compiler does any necessary arithmetic to produce 
> a pointer.  It's exactly as if I had written:
> 
>         spin_lock_irqsave(&mapping->pages.xa_lock, flags);
>         __delete_from_page_cache(page, NULL);
>         spin_unlock_irqrestore(&mapping->pages.xa_lock, 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 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. :)

--D

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