[Devel] Re: [PATCH] NFS: init client before declaration

[Stanislav Kinsbursky]

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 at netapp.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 at parallels.com>
> Signed-off-by: Trond Myklebust<Trond.Myklebust at netapp.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.


>   		nfs_put_client(clp);
>   		if (error)
>   			break;
> diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
> index b777bda..3be00a0 100644
> --- a/fs/nfs/internal.h
> +++ b/fs/nfs/internal.h
> @@ -168,6 +168,7 @@ extern struct nfs_server *nfs_clone_server(struct nfs_server *,
>   					   struct nfs_fattr *,
>   					   rpc_authflavor_t);
>   extern void nfs_mark_client_ready(struct nfs_client *clp, int state);
> +extern int nfs_wait_client_ready(struct nfs_client *clp);
>   extern int nfs4_check_client_ready(struct nfs_client *clp);
>   extern struct nfs_client *nfs4_set_ds_client(struct nfs_client* mds_clp,
>   					     const struct sockaddr *ds_addr,


-- 
Best regards,
Stanislav Kinsbursky