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

Yeah, that's neat.

Dealing with old compilers is frustrating...

--
Kirill A. Shutemov

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