From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mx1.redhat.com (mx1.redhat.com [209.132.183.28]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E217A1A045F for ; Thu, 27 Nov 2014 04:05:10 +1100 (AEDT) Date: Wed, 26 Nov 2014 19:04:47 +0200 From: "Michael S. Tsirkin" To: Christian Borntraeger Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic Message-ID: <20141126170447.GC11202@redhat.com> References: <1416915806-24757-1-git-send-email-dahi@linux.vnet.ibm.com> <20141126070258.GA25523@redhat.com> <20141126110504.511b733a@thinkpad-w530> <20141126151729.GB9612@redhat.com> <20141126152334.GA9648@redhat.com> <20141126163207.63810fcb@thinkpad-w530> <20141126154717.GB10568@redhat.com> <5475FAB1.1000802@de.ibm.com> <20141126163216.GB10850@redhat.com> <547604FC.4030300@de.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <547604FC.4030300@de.ibm.com> Cc: linux-arch@vger.kernel.org, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, David Hildenbrand , paulus@samba.org, schwidefsky@de.ibm.com, akpm@linux-foundation.org, linuxppc-dev@lists.ozlabs.org, mingo@kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , On Wed, Nov 26, 2014 at 05:51:08PM +0100, Christian Borntraeger wrote: > > But this one was > giving users in field false positives. > > So lets try to fix those, ok? If we cant, then tough luck. Sure. I think the simplest way might be to make spinlock disable premption when CONFIG_DEBUG_ATOMIC_SLEEP is enabled. As a result, userspace access will fail and caller will get a nice error. > But coming up with wrong statements is not helpful. True. Sorry that I did that. > > > > The point is that *_user is safe with preempt off. > > It returns an error gracefully. > > It does not sleep. > > It does not trigger the scheduler in that context. > > There are special cases where your statement is true. But its not in general. > copy_to_user might fault and that fault might sleep and reschedule. Yes. But not if called inatomic. > For example handle_mm_fault might go down to pud_alloc, pmd_alloc etc and all these functions could do an GFP_KERNEL allocation. Which might sleep. Which will schedule. > > > > > > > > David's patch makes it say it does, so it's wrong. > > > > > > Absolutely. I think you can already debug your case easily, by enabling CONFIG_PREEMPT. This seems counter-intuitive, and distro debug kernels don't seem to do this. -- MST