From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Tue, 08 Oct 2013 09:42:19 +0200 (CEST) Received: from localhost.localdomain ([127.0.0.1]:37799 "EHLO linux-mips.org" rhost-flags-OK-OK-OK-FAIL) by eddie.linux-mips.org with ESMTP id S6827313Ab3JHHmRdh0MN (ORCPT ); Tue, 8 Oct 2013 09:42:17 +0200 Received: from scotty.linux-mips.net (localhost.localdomain [127.0.0.1]) by scotty.linux-mips.net (8.14.7/8.14.4) with ESMTP id r987gFpT011866; Tue, 8 Oct 2013 09:42:15 +0200 Received: (from ralf@localhost) by scotty.linux-mips.net (8.14.7/8.14.7/Submit) id r987gFZo011865; Tue, 8 Oct 2013 09:42:15 +0200 Date: Tue, 8 Oct 2013 09:42:15 +0200 From: Ralf Baechle To: Tanguy Bouzeloc Cc: "linux-mips@linux-mips.org" Subject: Re: [PATCH] MIPS: fix forced successful syscalls Message-ID: <20131008074215.GG1615@linux-mips.org> References: <1380550969-9522-1-git-send-email-tanguy.bouzeloc@efixo.com> <20131002091909.GA23236@linux-mips.org> <524D84EB.8060102@efixo.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <524D84EB.8060102@efixo.com> User-Agent: Mutt/1.5.21 (2010-09-15) Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 38254 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: ralf@linux-mips.org Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On Thu, Oct 03, 2013 at 04:53:31PM +0200, Tanguy Bouzeloc wrote: > On 10/02/2013 11:19 AM, Ralf Baechle wrote: > >To my personal embarassment I have to admit that I knew about this since the > >day the syscall wrapper was written - but was considering it an acceptable > >bug ... > > > >Where it really bits is sigreturn and similar which use the following > >stunt: > > > > /* > > * Don't let your children do this ... > > */ > > __asm__ __volatile__( > > "move\t$29, %0\n\t" > > "j\tsyscall_exit" > > :/* no outputs */ > > :"r" (®s)); > > /* Unreached */ > > > >to keep the syscall return path from tampering with the return value. > > > >The scall*.S part of your patch is clearing TIF_NOERROR using a non-atomic > >LW/SW sequence. This needs to be done atomically or the thread's flags > >variable might get corrupted. This is complicated by MIPS I, R5900 and > >afair some older oddball not-quite MIPS II CPUs lacking LL/SC rsp. LLD/SCD. > > > > Ralf > > > > I discover the issue when changing the HZ of the kernel to 100HZ, in > this case the jiffies returned to the userland are the same as the > kernel ticks and it'll wrap after 5 minutes of uptime. With kernel > HZ at 250 or 1000H it'll make happen the ticks wrap after 230~260j. > > Unfortunately programs relying on ticks (they shouldn't but that > happens) have unpredictable behavior for 11.3s before the wrap. > > I can update the patch in order to access atomically the thread > flags, the point is ... it'll make the kernel incompatible with old > hardware. The price to pay: an ifdef ... #include ... #if (_MIPS_ISA > _MIPS_ISA_MIPS1) ... LL/SC code goes here #else ... LL/SC-less code goes here #endif But I'm wondering if we can move this hopefully relativly rare special case into do_notify_resume() and handle it in plain C there. Ralf