From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752552AbbCFN2l (ORCPT ); Fri, 6 Mar 2015 08:28:41 -0500 Received: from mx1.redhat.com ([209.132.183.28]:43598 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751910AbbCFN2i (ORCPT ); Fri, 6 Mar 2015 08:28:38 -0500 Date: Fri, 6 Mar 2015 14:26:34 +0100 From: Oleg Nesterov To: Ingo Molnar 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: <20150306132634.GA20693@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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150306075833.GA623@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: > > > [...] 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. > ======================> > From: Ingo Molnar > Date: Fri, 6 Mar 2015 08:37:57 +0100 > Subject: [PATCH] x86/fpu: Don't disable irqs in math_state_restore() > > math_state_restore() was historically called with irqs disabled, > because that's how the hardware generates the trap, and also because > back in the days it was possible for it to be an asynchronous > interrupt and interrupt handlers run with irqs off. > > These days it's always an instruction trap, and furthermore it does > inevitably complex things such as memory allocation and signal > processing, which is not done with irqs disabled. > > So keep irqs enabled. > > This might surprise in-kernel FPU users that somehow relied on > interrupts being disabled across FPU usage - but that's > fundamentally fragile anyway due to the inatomicity of FPU state > restores. The trap return will restore interrupts to its previous > state, but if FPU ops trigger math_state_restore() there's no > guarantee of atomicity anymore. > > To warn about in-kernel irqs-off users of FPU state we might want to > pass 'struct pt_regs' to math_state_restore() and check the trapped > state for irqs disabled (flags has IF cleared) and kernel context - > but that's for a later patch. > > Cc: Andy Lutomirski > Cc: Borislav Petkov > Cc: Fenghua Yu > Cc: H. Peter Anvin > Cc: Linus Torvalds > Cc: Oleg Nesterov > Cc: Quentin Casasnovas > Cc: Thomas Gleixner > Signed-off-by: Ingo Molnar > --- > arch/x86/kernel/traps.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c > index 950815a138e1..52f9e4057cee 100644 > --- a/arch/x86/kernel/traps.c > +++ b/arch/x86/kernel/traps.c > @@ -844,8 +844,9 @@ void math_state_restore(void) > { > struct task_struct *tsk = current; > > + local_irq_enable(); > + > if (!tsk_used_math(tsk)) { > - local_irq_enable(); > /* > * does a slab alloc which can sleep > */ > @@ -856,7 +857,6 @@ void math_state_restore(void) > do_group_exit(SIGKILL); > return; > } > - local_irq_disable(); > } > > /* Avoid __kernel_fpu_begin() right after __thread_fpu_begin() */