From mboxrd@z Thu Jan  1 00:00:00 1970
From: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH v5 03/78] xarray: Add the xa_lock to the radix_tree_root
Date: Tue, 26 Dec 2017 19:58:15 -0800
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To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: linux-kernel@vger.kernel.org, Matthew Wilcox <mawilcox@microsoft.com>, Ross Zwisler <ross.zwisler@linux.intel.com>, David Howells <dhowells@redhat.com>, Shaohua Li <shli@kernel.org>, Jens Axboe <axboe@kernel.dk>, Rehas Sachdeva <aquannie@gmail.com>, Marc Zyngier <marc.zyngier@arm.com>, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, linux-nilfs@vger.kernel.org, linux-btrfs@vger.kernel.org, linux-xfs@vger.kernel.org, linux-usb@vger.kernel.org, linux-raid@vger.kernel.org

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