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

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Message-ID: <20150316223345.GD29656@ZenIV.linux.org.uk>
Date:	Mon, 16 Mar 2015 22:33:45 +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 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()?
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