From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Tue, 2 Jan 2018 10:01:55 -0800 From: "Darrick J. Wong" To: Matthew Wilcox Cc: "Kirill A. Shutemov" , linux-kernel@vger.kernel.org, Matthew Wilcox , Ross Zwisler , David Howells , Shaohua Li , Jens Axboe , Rehas Sachdeva , Marc Zyngier , 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 Subject: Re: [PATCH v5 03/78] xarray: Add the xa_lock to the radix_tree_root Message-ID: <20180102180155.GD4857@magnolia> References: <20171215220450.7899-1-willy@infradead.org> <20171215220450.7899-4-willy@infradead.org> <20171226165440.tv6inwa2fgk3bfy6@node.shutemov.name> <20171227034340.GC24828@bombadil.infradead.org> <20171227035815.GD24828@bombadil.infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20171227035815.GD24828@bombadil.infradead.org> Sender: owner-linux-mm@kvack.org List-ID: 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 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 > -- > To unsubscribe from this list: send the line "unsubscribe linux-xfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org