net: fix races in page->_count manipulation

				goto end;

		}

		nc->frag.size = PAGE_SIZE << order;

recycle:

		atomic_set(&nc->frag.page->_count, NETDEV_PAGECNT_MAX_BIAS);

		/* Even if we own the page, we do not use atomic_set().

		 * This would break get_page_unless_zero() users.

		 */

		atomic_add(NETDEV_PAGECNT_MAX_BIAS - 1,

			   &nc->frag.page->_count);

		nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;

		nc->frag.offset = 0;

	}

	if (nc->frag.offset + fragsz > nc->frag.size) {

		/* avoid unnecessary locked operations if possible */

		if ((atomic_read(&nc->frag.page->_count) == nc->pagecnt_bias) ||

		    atomic_sub_and_test(nc->pagecnt_bias, &nc->frag.page->_count))

			goto recycle;

		if (atomic_read(&nc->frag.page->_count) != nc->pagecnt_bias) {

			if (!atomic_sub_and_test(nc->pagecnt_bias,

						 &nc->frag.page->_count))

				goto refill;

			/* OK, page count is 0, we can safely set it */

			atomic_set(&nc->frag.page->_count,

				   NETDEV_PAGECNT_MAX_BIAS);

		} else {

			atomic_add(NETDEV_PAGECNT_MAX_BIAS - nc->pagecnt_bias,

				   &nc->frag.page->_count);

		}

		nc->pagecnt_bias = NETDEV_PAGECNT_MAX_BIAS;

		nc->frag.offset = 0;

	}

	data = page_address(nc->frag.page) + nc->frag.offset;