From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: 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=-3.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=no 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 C10D2C43603 for ; Fri, 20 Dec 2019 20:58:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9E05A20866 for ; Fri, 20 Dec 2019 20:58:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727413AbfLTU6S convert rfc822-to-8bit (ORCPT ); Fri, 20 Dec 2019 15:58:18 -0500 Received: from albireo.enyo.de ([37.24.231.21]:56130 "EHLO albireo.enyo.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726525AbfLTU6S (ORCPT ); Fri, 20 Dec 2019 15:58:18 -0500 Received: from [172.17.203.2] (helo=deneb.enyo.de) by albireo.enyo.de with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) id 1iiPLh-000644-Fp; Fri, 20 Dec 2019 20:58:13 +0000 Received: from fw by deneb.enyo.de with local (Exim 4.92) (envelope-from ) id 1iiPKv-0003PZ-UB; Fri, 20 Dec 2019 21:57:25 +0100 From: Florian Weimer To: Mathieu Desnoyers Cc: Thomas Gleixner , linux-kernel , Peter Zijlstra , paulmck , Boqun Feng , "H. Peter Anvin" , Paul Turner , linux-api , stable , Dmitry Vyukov , Neel Natu Subject: Re: [PATCH for 5.5 1/2] rseq: Fix: Clarify rseq.h UAPI rseq_cs memory reclaim requirements References: <20191220201207.17389-1-mathieu.desnoyers@efficios.com> <87imman36g.fsf@mid.deneb.enyo.de> <173832695.14381.1576875253374.JavaMail.zimbra@efficios.com> Date: Fri, 20 Dec 2019 21:57:25 +0100 In-Reply-To: <173832695.14381.1576875253374.JavaMail.zimbra@efficios.com> (Mathieu Desnoyers's message of "Fri, 20 Dec 2019 15:54:13 -0500 (EST)") Message-ID: <875zian2a2.fsf@mid.deneb.enyo.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org * Mathieu Desnoyers: > ----- On Dec 20, 2019, at 3:37 PM, Florian Weimer fw@deneb.enyo.de wrote: > >> * Mathieu Desnoyers: >> >>> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h >>> index 9a402fdb60e9..6f26b0b148a6 100644 >>> --- a/include/uapi/linux/rseq.h >>> +++ b/include/uapi/linux/rseq.h >>> @@ -100,7 +100,9 @@ struct rseq { >>> * instruction sequence block, as well as when the kernel detects that >>> * it is preempting or delivering a signal outside of the range >>> * targeted by the rseq_cs. Also needs to be set to NULL by user-space >>> - * before reclaiming memory that contains the targeted struct rseq_cs. >>> + * before reclaiming memory that contains the targeted struct rseq_cs >>> + * or reclaiming memory that contains the code refered to by the >>> + * start_ip and post_commit_offset fields of struct rseq_cs. >> >> Maybe mention that it's good practice to clear rseq_cs before >> returning from a function that contains a restartable sequence? > > Unfortunately, clearing it is not free. Considering that rseq is meant to > be used in very hot code paths, it would be preferable that applications > clear it in the very infrequent case where the rseq_cs or code will > vanish (e.g. dlclose or JIT reclaim), and not require it to be cleared > after each critical section. I am therefore reluctant to document the > behavior you describe as a "good practice" for rseq. You already have to write to rseq_cs before entering the critical section, right? Then you've already determined the address, and the cache line is already hot, so it really should be close to zero cost. I mean, you can still discard the advice, but you do so ad your own peril …