linux-kernel - Re: [PATCH] NFS: init client before declaration

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Message-ID: <1337705000.4269.23.camel@lade.trondhjem.org>
Date:	Tue, 22 May 2012 16:43:23 +0000
From:	"Myklebust, Trond" <Trond.Myklebust@...app.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
CC:	"linux-nfs@...r.kernel.org" <linux-nfs@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...nvz.org" <devel@...nvz.org>
Subject: Re: [PATCH] NFS: init client before declaration

On Tue, 2012-05-22 at 20:18 +0400, Stanislav Kinsbursky wrote:
> On 22.05.2012 19:51, Myklebust, Trond wrote:
> > On Tue, 2012-05-22 at 19:29 +0400, Stanislav Kinsbursky wrote:
> >> On 22.05.2012 19:00, Myklebust, Trond wrote:
> >>> On Tue, 2012-05-22 at 10:29 -0400, Trond Myklebust wrote:
> >>>> On Tue, 2012-05-22 at 16:40 +0400, Stanislav Kinsbursky wrote:
> >>>>> Client have to be initialized prior to adding it to per-net clients list,
> >>>>> because otherwise there are races, shown below:
> >>>>>
> >>>>> CPU#0					CPU#1
> >>>>> _____					_____
> >>>>>
> >>>>> nfs_get_client
> >>>>> nfs_alloc_client
> >>>>> list_add(..., nfs_client_list)
> >>>>> 					rpc_fill_super
> >>>>> 					rpc_pipefs_event
> >>>>> 					nfs_get_client_for_event
> >>>>> 					__rpc_pipefs_event
> >>>>> 					(clp->cl_rpcclient is uninitialized)
> >>>>> 					BUG()
> >>>>> init_client
> >>>>> clp->cl_rpcclient = ...
> >>>>>
> >>>>
> >>>> Why not simply change nfs_get_client_for_event() so that it doesn't
> >>>> touch nfs_clients that have clp->cl_cons_state!=NFS_CS_READY?
> >>>>
> >>>> That should ensure that it doesn't touch nfs_clients that failed to
> >>>> initialise and/or are still in the process of being initialised.
> >>>
> >>> ...actually, come to think of it. Why not just add a helper function
> >>> "bool nfs_client_active(const struct nfs_client *clp)" to
> >>> fs/nfs/client.c that does a call to
> >>> 	wait_event_killable(nfs_client_active_wq, clp->cl_cons_state<   NFS_CS_INITING);
> >>> and checks the resulting value of clp->cl_cons_state?
> >>>
> >>
> >> Sorry, but I don't understand the idea...
> >> Where are you proposing to call this function?
> >> In __rpc_pipefs_event() prior to dentries creatios?
> >
> > See below:
> >
> > 8<----------------------------------------------------------------------------------
> >  From f5b90df6381a20395d9f88a199e9e52f44267457 Mon Sep 17 00:00:00 2001
> > From: Trond Myklebust<Trond.Myklebust@...app.com>
> > Date: Tue, 22 May 2012 11:49:55 -0400
> > Subject: [PATCH] NFSv4: Fix a race in the net namespace mount notification
> >
> > Since the struct nfs_client gets added to the global nfs_client_list
> > before it is initialised, it is possible that rpc_pipefs_event can
> > end up trying to create idmapper entries for such a thing.
> >
> > The solution is to have the mount notification wait for the
> > nfs_client initialisation to complete.
> >
> > Reported-by: Stanislav Kinsbursky<skinsbursky@...allels.com>
> > Signed-off-by: Trond Myklebust<Trond.Myklebust@...app.com>
> > ---
> >   fs/nfs/client.c   |   14 ++++++++++++++
> >   fs/nfs/idmap.c    |    3 ++-
> >   fs/nfs/internal.h |    1 +
> >   3 files changed, 17 insertions(+), 1 deletions(-)
> >
> > diff --git a/fs/nfs/client.c b/fs/nfs/client.c
> > index 60f7e4e..3fa44ef 100644
> > --- a/fs/nfs/client.c
> > +++ b/fs/nfs/client.c
> > @@ -592,6 +592,20 @@ void nfs_mark_client_ready(struct nfs_client *clp, int state)
> >   	wake_up_all(&nfs_client_active_wq);
> >   }
> >
> > +static bool nfs_client_ready(struct nfs_client *clp)
> > +{
> > +	return clp->cl_cons_state<= NFS_CS_READY;
> > +}
> > +
> > +int nfs_wait_client_ready(struct nfs_client *clp)
> > +{
> > +	if (wait_event_killable(nfs_client_active_wq, nfs_client_ready(clp))<  0)
> > +		return -ERESTARTSYS;
> 
> Ok, I see...
> BTW, caller of this function is pipefs mount operation call... And when this 
> mount call waits for NFS clients - it look a bit odd to me...
> 
> 
> > +	if (clp->cl_cons_state<  0)
> > +		return clp->cl_cons_state;
> > +	return 0;
> > +}
> > +
> >   /*
> >    * With sessions, the client is not marked ready until after a
> >    * successful EXCHANGE_ID and CREATE_SESSION.
> > diff --git a/fs/nfs/idmap.c b/fs/nfs/idmap.c
> > index 3e8edbe..67962c8 100644
> > --- a/fs/nfs/idmap.c
> > +++ b/fs/nfs/idmap.c
> > @@ -558,7 +558,8 @@ static int rpc_pipefs_event(struct notifier_block *nb, unsigned long event,
> >   		return 0;
> >
> >   	while ((clp = nfs_get_client_for_event(sb->s_fs_info, event))) {
> > -		error = __rpc_pipefs_event(clp, event, sb);
> > +		if (nfs_wait_client_ready(clp) == 0)
> > +			error = __rpc_pipefs_event(clp, event, sb);
> 
> 
> We have another problem here.
> nfs4_init_client() will try to create pipe dentries prior to set of NFS_CS_READY 
> to the client. And dentries will be created since semaphore is dropped and 
> per-net superblock variable is initialized already.
> But __rpc_pipefs_event() relays on the fact, that no dentries present.
> Looks like the problem was introduced by me in aad9487c...
> So maybe we should not call "continue" instead "__rpc_pipefs_event()", when 
> client becomes ready?
> Looks like this will allow us to handle such races.

Let me rework this patch a bit...

-- 
Trond Myklebust
Linux NFS client maintainer

NetApp
Trond.Myklebust@...app.com
www.netapp.com