Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751038AbdFUItR (ORCPT ); Wed, 21 Jun 2017 04:49:17 -0400 Received: from mail.skyhub.de ([5.9.137.197]:50410 "EHLO mail.skyhub.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751093AbdFUItO (ORCPT ); Wed, 21 Jun 2017 04:49:14 -0400 Date: Wed, 21 Jun 2017 10:49:02 +0200 From: Borislav Petkov To: Andy Lutomirski Cc: x86@kernel.org, linux-kernel@vger.kernel.org, Linus Torvalds , Andrew Morton , Mel Gorman , "linux-mm@kvack.org" , Nadav Amit , Rik van Riel , Dave Hansen , Arjan van de Ven , Peter Zijlstra Subject: Re: [PATCH v3 01/11] x86/mm: Don't reenter flush_tlb_func_common() Message-ID: <20170621084902.vy7nvkon4krc7v3q@pd.tnic> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2986 Lines: 81 On Tue, Jun 20, 2017 at 10:22:07PM -0700, Andy Lutomirski wrote: > It was historically possible to have two concurrent TLB flushes > targetting the same CPU: one initiated locally and one initiated > remotely. This can now cause an OOPS in leave_mm() at > arch/x86/mm/tlb.c:47: > > if (this_cpu_read(cpu_tlbstate.state) == TLBSTATE_OK) > BUG(); > > with this call trace: > flush_tlb_func_local arch/x86/mm/tlb.c:239 [inline] > flush_tlb_mm_range+0x26d/0x370 arch/x86/mm/tlb.c:317 These line numbers would most likely mean nothing soon. I think you should rather explain why the bug can happen so that future lookers at that code can find the spot... > > Without reentrancy, this OOPS is impossible: leave_mm() is only > called if we're not in TLBSTATE_OK, but then we're unexpectedly > in TLBSTATE_OK in leave_mm(). > > This can be caused by flush_tlb_func_remote() happening between > the two checks and calling leave_mm(), resulting in two consecutive > leave_mm() calls on the same CPU with no intervening switch_mm() > calls. ...like this, for example. That should be more future-code-changes-proof. > We never saw this OOPS before because the old leave_mm() > implementation didn't put us back in TLBSTATE_OK, so the assertion > didn't fire. > > Nadav noticed the reentrancy issue in a different context, but > neither of us realized that it caused a problem yet. > > Cc: Nadav Amit > Cc: Dave Hansen > Reported-by: "Levin, Alexander (Sasha Levin)" > Fixes: 3d28ebceaffa ("x86/mm: Rework lazy TLB to track the actual loaded mm") > Signed-off-by: Andy Lutomirski > --- > arch/x86/mm/tlb.c | 15 +++++++++++++-- > 1 file changed, 13 insertions(+), 2 deletions(-) > > diff --git a/arch/x86/mm/tlb.c b/arch/x86/mm/tlb.c > index 2a5e851f2035..f06239c6919f 100644 > --- a/arch/x86/mm/tlb.c > +++ b/arch/x86/mm/tlb.c > @@ -208,6 +208,9 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next, > static void flush_tlb_func_common(const struct flush_tlb_info *f, > bool local, enum tlb_flush_reason reason) > { > + /* This code cannot presently handle being reentered. */ > + VM_WARN_ON(!irqs_disabled()); > + > if (this_cpu_read(cpu_tlbstate.state) != TLBSTATE_OK) { > leave_mm(smp_processor_id()); > return; > @@ -313,8 +316,12 @@ void flush_tlb_mm_range(struct mm_struct *mm, unsigned long start, > info.end = TLB_FLUSH_ALL; > } > > - if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) > + if (mm == this_cpu_read(cpu_tlbstate.loaded_mm)) { > + local_irq_disable(); > flush_tlb_func_local(&info, TLB_LOCAL_MM_SHOOTDOWN); > + local_irq_enable(); > + } I'm assuming this is going away in a future patch, as disabling IRQs around a TLB flush is kinda expensive. I guess I'll see if I continue reading... :) -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.