From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e06smtp10.uk.ibm.com (e06smtp10.uk.ibm.com [195.75.94.106]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id EAB1C1A0519 for ; Thu, 27 Nov 2014 03:02:46 +1100 (AEDT) Received: from /spool/local by e06smtp10.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 26 Nov 2014 16:02:43 -0000 Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197]) by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id E24321B0805F for ; Wed, 26 Nov 2014 16:02:54 +0000 (GMT) Received: from d06av01.portsmouth.uk.ibm.com (d06av01.portsmouth.uk.ibm.com [9.149.37.212]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id sAQG2dFE3473896 for ; Wed, 26 Nov 2014 16:02:39 GMT Received: from d06av01.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av01.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id sAQG2ahN020824 for ; Wed, 26 Nov 2014 09:02:38 -0700 Message-ID: <5475F99B.3090307@de.ibm.com> Date: Wed, 26 Nov 2014 17:02:35 +0100 From: Christian Borntraeger MIME-Version: 1.0 To: "Michael S. Tsirkin" Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic References: <1416915806-24757-1-git-send-email-dahi@linux.vnet.ibm.com> <20141126070258.GA25523@redhat.com> <20141126110504.511b733a@thinkpad-w530> <20141126151729.GB9612@redhat.com> <5475F218.4050207@de.ibm.com> <20141126153732.GA10568@redhat.com> In-Reply-To: <20141126153732.GA10568@redhat.com> Content-Type: text/plain; charset=windows-1252 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: , Am 26.11.2014 um 16:37 schrieb Michael S. Tsirkin: > On Wed, Nov 26, 2014 at 04:30:32PM +0100, Christian Borntraeger wrote: >> Am 26.11.2014 um 16:17 schrieb Michael S. Tsirkin: >>> On Wed, Nov 26, 2014 at 11:05:04AM +0100, David Hildenbrand wrote: >>>>> What's the path you are trying to debug? >>>> >>>> Well, we had a problem where we held a spin_lock and called >>>> copy_(from|to)_user(). We experienced very random deadlocks that took some guy >>>> almost a week to debug. The simple might_sleep() check would have showed this >>>> error immediately. >>> >> >>> This must have been a very old kernel. >>> A modern kernel will return an error from copy_to_user. >> >> I disagree. copy_to_user will not return while holding a spinlock, because it does not know! How should it? >> See: spin_lock will call preempt_disable, but thats a no-op for a non-preempt kernel. So the mere fact that we hold a spin_lock is not known by any user access function. (or others). No? >> >> Christian >> >> > > Well might_sleep() merely checks preempt count and irqs_disabled too. > If you want debugging things to trigger, you need to enable > a bunch of config options. That's not new. You miss the point of the whole thread: The problem is that even with debug options enabled, holding a spinlock would not trigger a bug on copy_to_user. So the problem is not the good path, the problem is that a debugging aid for detecting a broken case was lost. Even with all kernel debugging enabled. That is because CONFIG_DEBUG_ATOMIC_SLEEP selects PREEMPT_COUNT. That means: spin_lock will then be considered as in_atomic and no message comes. Without CONFIG_DEBUG_ATOMIC_SLEEP spin_lock will not touch the preempt_count but we also dont see a message because might_fault is now a nop I understand that you dont like Davids changes due to other side effects that you have mentioned. So lets focus on how we can fix the debug option. Ok? Christian