NFS: Fix race in nfs_set_page_dirty

Protect nfs_set_page_dirty() against races with nfs_inode_add_request.
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

fs/nfs/write.c

@@ -388,6 +388,8 @@ static int nfs_inode_add_request(struct inode *inode, struct nfs_page *req)
 	}
 	SetPagePrivate(req->wb_page);
 	set_page_private(req->wb_page, (unsigned long)req);
+	if (PageDirty(req->wb_page))
+		set_bit(PG_NEED_FLUSH, &req->wb_flags);
 	nfsi->npages++;
 	atomic_inc(&req->wb_count);
 	return 0;
@@ -407,6 +409,8 @@ static void nfs_inode_remove_request(struct nfs_page *req)
 	set_page_private(req->wb_page, 0);
 	ClearPagePrivate(req->wb_page);
 	radix_tree_delete(&nfsi->nfs_page_tree, req->wb_index);
+	if (test_and_clear_bit(PG_NEED_FLUSH, &req->wb_flags))
+		__set_page_dirty_nobuffers(req->wb_page);
 	nfsi->npages--;
 	if (!nfsi->npages) {
 		spin_unlock(&nfsi->req_lock);
@@ -1527,15 +1531,22 @@ int nfs_wb_page(struct inode *inode, struct page* page)
 
 int nfs_set_page_dirty(struct page *page)
 {
+	spinlock_t *req_lock = &NFS_I(page->mapping->host)->req_lock;
 	struct nfs_page *req;
+	int ret;
 
-	req = nfs_page_find_request(page);
+	spin_lock(req_lock);
+	req = nfs_page_find_request_locked(page);
 	if (req != NULL) {
 		/* Mark any existing write requests for flushing */
-		set_bit(PG_NEED_FLUSH, &req->wb_flags);
+		ret = !test_and_set_bit(PG_NEED_FLUSH, &req->wb_flags);
+		spin_unlock(req_lock);
 		nfs_release_request(req);
+		return ret;
 	}
-	return __set_page_dirty_nobuffers(page);
+	ret = __set_page_dirty_nobuffers(page);
+	spin_unlock(req_lock);
+	return ret;
 }