From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754117AbbCFOEI (ORCPT ); Fri, 6 Mar 2015 09:04:08 -0500 Received: from mx1.redhat.com ([209.132.183.28]:44788 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752881AbbCFOED (ORCPT ); Fri, 6 Mar 2015 09:04:03 -0500 Date: Fri, 6 Mar 2015 15:01:54 +0100 From: Oleg Nesterov To: Ingo Molnar , David Vrabel Cc: Dave Hansen , Borislav Petkov , Andy Lutomirski , Linus Torvalds , Pekka Riikonen , Rik van Riel , Suresh Siddha , LKML , "Yu, Fenghua" , Quentin Casasnovas Subject: Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs Message-ID: <20150306140154.GA22811@redhat.com> References: <54F74F59.5070107@intel.com> <20150305195127.GA12657@redhat.com> <20150305195149.GB12657@redhat.com> <20150305201101.GA21571@gmail.com> <20150305212532.GA16890@redhat.com> <20150306075833.GA623@gmail.com> <20150306132634.GA20693@redhat.com> <20150306134601.GA11718@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150306134601.GA11718@gmail.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 03/06, Ingo Molnar wrote: > > * Oleg Nesterov wrote: > > > On 03/06, Ingo Molnar wrote: > > > > > > * Oleg Nesterov wrote: > > > > > > > [...] The patch above looks "obviously safe", but perhaps I am > > > > paranoid too much... > > > > > > IMHO your hack above isn't really acceptable, even for a backport. > > > So lets test the patch below (assuming it's the right thing to do) > > > and move forward? > > > > OK, but please note that this patch is not beckportable. If you think > > that -stable doesn't need this fix, then I agree. > > > > If the caller is do_device_not_available(), then we can not enable > > irqs before __thread_fpu_begin() + restore_fpu_checking(). > > > > 1. Preemption in between can destroy ->fpu.state initialized by > > fpu_finit(), __switch_to() will save the live (wrong) FPU state > > again. > > > > 2. kernel_fpu_begin() from irq right after __thread_fpu_begin() is > > not nice too. It will do __save_init_fpu() and this overwrites > > ->fpu.state too. > > > > Starting from v4.0 it does kernel_fpu_disable(), but the older kernels > > do not. > > > > Ingo, this code is really horrible and fragile. We need to cleanup it > > step-by-step, imho. > > How about the patch from David Vrabel? That seems to solve the > irq-disable problem too, right? I wasn't cc'ed, I guess you mean [PATCHv4] x86, fpu: remove the logic of non-eager fpu mem allocation at the first usage http://marc.info/?l=linux-kernel&m=142564237705311&w=2 Not sure I understand it correctly after the first quick look, but 1. It conflicts with the recent changes in tip/x86/fpu 2. fpu_ini() initializes current->thread.fpu.state. This looks unneeded, the kernel threads no longer have FPU context and do not abuse CPU. 3. I can be easily wrong, but it looks buggy... Note that arch_dup_task_struct() doesn't allocate child->fpu.state if !tsk_used_math(parent). Add David... No, I do not think this patch is a good idea. Perhaps I am wrong, but I think we need other changes. And they should start from init_fpu(). Oleg.