From: "J. Bruce Fields" Subject: Re: [PATCH 1/4] knfsd: Replace lock_kernel with a mutex for nfsd thread startup/shutdown locking. Date: Wed, 4 Jun 2008 17:58:15 -0400 Message-ID: <20080604215815.GF10362@fieldses.org> References: <1212591796-22144-1-git-send-email-jlayton@redhat.com> <1212591796-22144-2-git-send-email-jlayton@redhat.com> <20080604210235.GC10362@fieldses.org> <20080604172752.31686797@tleilax.poochiereds.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-nfs@vger.kernel.org, nfsv4@linux-nfs.org, gnb-cP1dWloDopni96+mSzHFpQC/G2K4zDHf@public.gmane.org To: Jeff Layton Return-path: Received: from mail.fieldses.org ([66.93.2.214]:57682 "EHLO fieldses.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751991AbYFDV6R (ORCPT ); Wed, 4 Jun 2008 17:58:17 -0400 In-Reply-To: <20080604172752.31686797-RtJpwOs3+0O+kQycOl6kW4xkIHaj4LzF@public.gmane.org> Sender: linux-nfs-owner@vger.kernel.org List-ID: On Wed, Jun 04, 2008 at 05:27:52PM -0400, Jeff Layton wrote: > On Wed, 4 Jun 2008 17:02:35 -0400 > "J. Bruce Fields" wrote: > > > On Wed, Jun 04, 2008 at 11:03:13AM -0400, Jeff Layton wrote: > > > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c > > > index 5ac00c4..d601a77 100644 > > > --- a/fs/nfsd/nfsctl.c > > > +++ b/fs/nfsd/nfsctl.c > > ... > > > @@ -566,14 +574,13 @@ static ssize_t write_versions(struct file *file, char *buf, size_t size) > > > return len; > > > } > > > > > > -static ssize_t write_ports(struct file *file, char *buf, size_t size) > > > +static ssize_t __write_ports(struct file *file, char *buf, size_t size) > > > { > > > if (size == 0) { > > > int len = 0; > > > - lock_kernel(); > > > + > > > if (nfsd_serv) > > > len = svc_xprt_names(nfsd_serv, buf, 0); > > > - unlock_kernel(); > > > > svc_xprt_names() has to be prepared to accept NULL as a first parameter > > (since we've got nothing here any longer to guarantee that nfsd_serv > > won't change after we've checked it). And, indeed, it does check for > > that (with its local copy, which won't change. So that's OK. But then > > could we just ditch this redundant check here? It's confusing. > > > > Oops, but: what happens if something like this races with svc_destroy, > > so svc_xprt_names() is passed a pointer to freed memory? > > > > We do have a guarantee that nfsd_serv won't change after it's checked > here. The new nfsd_mutex protects it. write_ports has been renamed to > __write_ports, and write_ports has been turned into a wrapper that runs > the entire original function under the nfsd_mutex. We also have nfsd > hold the nfsd_mutex when svc_exit_thread is called, so svc_destroy > should also be called while holding it. That should serialize access > to the nfsd_serv. Of course, you're right; thanks for setting me straight! > > I think you're correct that we can get rid of the redundant null > pointer check in __write_ports here though. OK.--b.