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