Re: [PATCH v2 1/4] SUNRPC: release per-net clients lock before calling PipeFS dentries creation

["Myklebust, Trond" <Trond.Myklebust@netapp.com>]

On Mon, 2012-02-27 at 19:50 +0400, Stanislav Kinsbursky wrote:
> Lockdep is sad otherwise, because inode mutex is taken on PipeFS dentry
> creation, which can be called on mount notification, where this per-net client
> lock is taken on clients list walk.
> 
> Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
> 
> ---
>  net/sunrpc/clnt.c |   10 +++++++---
>  1 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/net/sunrpc/clnt.c b/net/sunrpc/clnt.c
> index bb7ed2f3..ddb5741 100644
> --- a/net/sunrpc/clnt.c
> +++ b/net/sunrpc/clnt.c
> @@ -84,7 +84,7 @@ static void rpc_register_client(struct rpc_clnt *clnt)
>  	struct sunrpc_net *sn = net_generic(clnt->cl_xprt->xprt_net, sunrpc_net_id);
>  
>  	spin_lock(&sn->rpc_client_lock);
> -	list_add(&clnt->cl_clients, &sn->all_clients);
> +	list_add_tail(&clnt->cl_clients, &sn->all_clients);
>  	spin_unlock(&sn->rpc_client_lock);
>  }
>  
> @@ -208,15 +208,19 @@ static int rpc_pipefs_event(struct notifier_block *nb, unsigned long event,
>  			    void *ptr)
>  {
>  	struct super_block *sb = ptr;
> -	struct rpc_clnt *clnt;
> +	struct rpc_clnt *clnt, *tmp;
>  	int error = 0;
>  	struct sunrpc_net *sn = net_generic(sb->s_fs_info, sunrpc_net_id);
>  
>  	spin_lock(&sn->rpc_client_lock);
> -	list_for_each_entry(clnt, &sn->all_clients, cl_clients) {
> +	list_for_each_entry_safe(clnt, tmp, &sn->all_clients, cl_clients) {
> +		atomic_inc(&clnt->cl_count);
> +		spin_unlock(&sn->rpc_client_lock);
>  		error = __rpc_pipefs_event(clnt, event, sb);
> +		rpc_release_client(clnt);
>  		if (error)
>  			break;
> +		spin_lock(&sn->rpc_client_lock);
>  	}
>  	spin_unlock(&sn->rpc_client_lock);
>  	return error;
> 

This won't be safe. Nothing guarantees that 'tmp' remains valid after
you drop the spin_lock.

I think you rather need to add a check for whether clnt->cl_dentry is in
the right state (NULL if RPC_PIPEFS_UMOUNT or non-NULL if
RPC_PIPEFS_MOUNT) before deciding whether or not to atomic_inc() and
drop the lock, so that you can restart the loop after calling
__rpc_pipefs_event().


-- 
Trond Myklebust
Linux NFS client maintainer

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