linux-kernel - Re: [PATCH v2 00/16] nfsd/sunrpc: add support for a workqueue-based nfsd

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Date:	Fri, 12 Dec 2014 16:59:52 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jeff Layton <jeff.layton@...marydata.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Bruce Fields <bfields@...ldses.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linux NFS Mailing List <linux-nfs@...r.kernel.org>,
	Tejun Heo <tj@...nel.org>, NeilBrown <neilb@...e.de>
Subject: Re: [PATCH v2 00/16] nfsd/sunrpc: add support for a workqueue-based
 nfsd

On Fri, Dec 12, 2014 at 06:54:08AM -0500, Jeff Layton wrote:

> > Umm...  I would be very surprised if it turned out to be a problem.
> > nfsd really doesn't give a fuck about its cwd and root - not in the
> > thread side of things.  And (un)exporting is (a) not on a hot path
> > and (b) not done from a kernel thread anyway.  fh_to_dentry and friends
> > doesn't care about root/cwd, etc.
> > 
> > I don't see anything that could cause that kind of issues.
> 
> I like the change overall -- it would certainly make my patch series
> simpler, but what about pathwalking? We do take the fs->lock in
> unlazy_walk. Is it possible we'd end up with more contention there?

That would take a pathname lookup in kernel thread side of nfsd that
	* isn't single-component
	* isn't LOOKUP_ROOT one (i.e. vfs_path_lookup() or file_open_root())
and I would really hope we don't have such things.  Any such a beast would
allow probing the tree layout outside of what we export, to start with...

AFAICS, we really don't have anything of that sort.  Note that e.g.
lookup_one_len() doesn't go anywhere near ->fs->lock...
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