linux-kernel - Re: [git pull] more vfs bits

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Message-ID: <27542.1424609052@warthog.procyon.org.uk>
Date:	Sun, 22 Feb 2015 12:44:12 +0000
From:	David Howells <dhowells@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	dhowells@...hat.com, Al Viro <viro@...iv.linux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [git pull] more vfs bits

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> So the ACCESS_ONCE() thing is more special than just "done under RCU".
> It's more like "really special case done without any of the normal
> locking _or_ any of the normal RCU checks".
> 
> That said, the overhead of using ACCESS_ONCE() is basically nil, so
> it's not like we couldn't just start doing more of them, and make it
> be more of a "any time we're under RCU" kind of thing.

Some functions access ->d_inode more than once.  Wouldn't that potentially
increase the number of load instructions?  Admittedly, calls to
dentry->d_inode could be replaced with inode = dentry->d_inode, then use
inode.

> Yeah, I think "d_backing_store_inode()" would probably be more along
> the lines, but that's a mouthful. Maybe shortened to
> "d_backing_inode()"?

Sounds more reasonable than d_opened_inode().  d_actual_inode() might also
work.  d_lower_inode() might work too.

David
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