linux-kernel - Re: [PATCH v3 1/1] dcache: Translating dentry into pathname without taking rename

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Message-ID: <CA+55aFzf-reQNSQt_fhHfUe_tB19n2qEa=Rd=wFEBJEVALmezA@mail.gmail.com>
Date:	Fri, 6 Sep 2013 14:48:32 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Viro <viro@...iv.linux.org.uk>
Cc:	Waiman Long <Waiman.Long@...com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	George Spelvin <linux@...izon.com>,
	John Stoffel <john@...ffel.org>
Subject: Re: [PATCH v3 1/1] dcache: Translating dentry into pathname without
 taking rename_lock

On Fri, Sep 6, 2013 at 2:05 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
> I can take that, but I'm really not convinced that we need writer lock
> there at all.  After all, if we really can get livelocks on that one,
> we would be getting them on d_lookup()...

d_lookup() does a _single_ path component. That's a *big* difference.

Sure, the hash chain that d_lookup() (well, __d_lookup()) ends up
walking is a bit more complicated than just following the dentry
parent pointer, but that's much harder to trigger than just creating a
really deep directory structure of single-letter nested directories,
and then doing a "getcwd()" that walks 1024+ parents, while another
thread is looping renaming things..

So I personally do feel a lot safer with the fallback to write locking here.

Especially since it's pretty simple, so there isn't really much downside.

             Linus
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