NFS: Fix put_nfs_open_context

We need to grab the inode->i_lock atomically with the last reference put in
order to remove the open context that is being freed from the
nfsi->open_files list.

Fix by converting the kref to a standard atomic counter and then using
atomic_dec_and_lock()...

Thanks to Arnd Bergmann for pointing out the problem.
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>

fs/nfs/inode.c

@@ -468,7 +468,7 @@ static struct nfs_open_context *alloc_nfs_open_context(struct vfsmount *mnt, str
 		ctx->lockowner = current->files;
 		ctx->error = 0;
 		ctx->dir_cookie = 0;
-		kref_init(&ctx->kref);
+		atomic_set(&ctx->count, 1);
 	}
 	return ctx;
 }
@@ -476,21 +476,18 @@ static struct nfs_open_context *alloc_nfs_open_context(struct vfsmount *mnt, str
 struct nfs_open_context *get_nfs_open_context(struct nfs_open_context *ctx)
 {
 	if (ctx != NULL)
-		kref_get(&ctx->kref);
+		atomic_inc(&ctx->count);
 	return ctx;
 }
 
-static void nfs_free_open_context(struct kref *kref)
+void put_nfs_open_context(struct nfs_open_context *ctx)
 {
-	struct nfs_open_context *ctx = container_of(kref,
-			struct nfs_open_context, kref);
-
-	if (!list_empty(&ctx->list)) {
 	struct inode *inode = ctx->path.dentry->d_inode;
-		spin_lock(&inode->i_lock);
+
+	if (!atomic_dec_and_lock(&ctx->count, &inode->i_lock))
+		return;
 	list_del(&ctx->list);
 	spin_unlock(&inode->i_lock);
-	}
 	if (ctx->state != NULL)
 		nfs4_close_state(&ctx->path, ctx->state, ctx->mode);
 	if (ctx->cred != NULL)
@@ -500,11 +497,6 @@ static void nfs_free_open_context(struct kref *kref)
 	kfree(ctx);
 }
 
-void put_nfs_open_context(struct nfs_open_context *ctx)
-{
-	kref_put(&ctx->kref, nfs_free_open_context);
-}
-
 /*
  * Ensure that mmap has a recent RPC credential for use when writing out
  * shared pages

include/linux/nfs_fs.h

@@ -71,7 +71,7 @@ struct nfs_access_entry {
 
 struct nfs4_state;
 struct nfs_open_context {
-	struct kref kref;
+	atomic_t count;
 	struct path path;
 	struct rpc_cred *cred;
 	struct nfs4_state *state;