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:

quoted

    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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