linux-kernel - Re: [PATCH 06/13] VFS/namei: new flag to support RCU symlinks: LOOKUP_LINK

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Message-ID: <20150317005944.GI29656@ZenIV.linux.org.uk>
Date:	Tue, 17 Mar 2015 00:59:44 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	NeilBrown <neilb@...e.de>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 06/13] VFS/namei: new flag to support RCU symlinks:
 LOOKUP_LINK_RCU.

On Mon, Mar 16, 2015 at 10:33:45PM +0000, Al Viro wrote:
> On Mon, Mar 16, 2015 at 03:43:20PM +1100, NeilBrown wrote:
> > When we support ->follow_link in RCU-walk we will not want to
> > take a reference to the 'struct path *link' passed to follow_link,
> > and correspondingly will not want to drop that reference.
> > 
> > As link_path_walk will complete_walk() in the case of an error,
> > and as complete_walk() will clear LOOKUP_RCU, we cannot test
> > LOOKUP_RCU to determine if the path should be 'put'.
> > 
> > So introduce a new flag: LOOKUP_LINK_RCU.  This is set on
> > entry to follow_link() if appropriate and put_link() will
> > only call path_put() if it is clear.
> 
> Umm...  How is it different from nd->depth > 0 && nd->flags & LOOKUP_RCU?
> IOW, could we bump nd->depth before that (conditional) mntget()?

OK, I see...  So you are holding that flag for as long as we are traversing
any part of a symlink body, including that of a trailing symlink...
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