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

[Matthew Wilcox]

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