From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=c+yg=TC=vger.kernel.org=linux-kernel-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no
	version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 730ACC43219
	for <linux-kernel@archiver.kernel.org>; Thu,  2 May 2019 15:46:53 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id 4BBDE204EC
	for <linux-kernel@archiver.kernel.org>; Thu,  2 May 2019 15:46:53 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1727535AbfEBPqw (ORCPT
        <rfc822;linux-kernel@archiver.kernel.org>);
        Thu, 2 May 2019 11:46:52 -0400
Received: from zeniv.linux.org.uk ([195.92.253.2]:52424 "EHLO
        ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1726896AbfEBPqu (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Thu, 2 May 2019 11:46:50 -0400
Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92 #3 (Red Hat Linux))
        id 1hMDv2-0001VL-TM; Thu, 02 May 2019 15:46:44 +0000
Date:   Thu, 2 May 2019 16:46:44 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Kees Cook <keescook@chromium.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org
Subject: Re: Alloc refcount increments to fail
Message-ID: <20190502154644.GV23075@ZenIV.linux.org.uk>
References: <20190502152621.GB18948@bombadil.infradead.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190502152621.GB18948@bombadil.infradead.org>
User-Agent: Mutt/1.11.3 (2019-02-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Thu, May 02, 2019 at 08:26:21AM -0700, Matthew Wilcox wrote:

> +/**
> + * refcount_try_inc - Increment a refcount if it's below INT_MAX
> + * @r: the refcount to increment
> + *
> + * Avoid the counter saturating by declining to increment the counter
> + * if it is more than halfway to saturation.
> + */
> +static inline __must_check bool refcount_try_inc(refcount_t *r)
> +{
> +	if (refcount_read(r) < 0)
> +		return false;
> +	refcount_inc(r);
> +	return true;
> +}

So two of those in parallel with have zero protection, won't they?