From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from e06smtp15.uk.ibm.com (e06smtp15.uk.ibm.com [195.75.94.111]) (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id B9FDE1A00A2 for ; Thu, 27 Nov 2014 03:07:22 +1100 (AEDT) Received: from /spool/local by e06smtp15.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 26 Nov 2014 16:07:18 -0000 Received: from b06cxnps4075.portsmouth.uk.ibm.com (d06relay12.portsmouth.uk.ibm.com [9.149.109.197]) by d06dlp01.portsmouth.uk.ibm.com (Postfix) with ESMTP id 0A29B17D805A for ; Wed, 26 Nov 2014 16:07:31 +0000 (GMT) Received: from d06av07.portsmouth.uk.ibm.com (d06av07.portsmouth.uk.ibm.com [9.149.37.248]) by b06cxnps4075.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id sAQG7FFk65142892 for ; Wed, 26 Nov 2014 16:07:15 GMT Received: from d06av07.portsmouth.uk.ibm.com (localhost [127.0.0.1]) by d06av07.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id sAQG7Eqw009798 for ; Wed, 26 Nov 2014 11:07:15 -0500 Message-ID: <5475FAB1.1000802@de.ibm.com> Date: Wed, 26 Nov 2014 17:07:13 +0100 From: Christian Borntraeger MIME-Version: 1.0 To: "Michael S. Tsirkin" , David Hildenbrand 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> <20141126152334.GA9648@redhat.com> <20141126163207.63810fcb@thinkpad-w530> <20141126154717.GB10568@redhat.com> In-Reply-To: <20141126154717.GB10568@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, 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 16:47 schrieb Michael S. Tsirkin: > On Wed, Nov 26, 2014 at 04:32:07PM +0100, David Hildenbrand wrote: >>> On Wed, Nov 26, 2014 at 05:17:29PM +0200, Michael S. Tsirkin wrote: >>>> 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. >>>> Which is really the point of the patch you are trying to revert. >>> >>> That's assuming you disabled preemption. If you didn't, and take >>> a spinlock, you have deadlocks even without userspace access. >>> >> >> (Thanks for your resent, my first email was sent directly to you ... grml) >> >> This is what happened on our side (very recent kernel): >> >> spin_lock(&lock) >> copy_to_user(...) >> spin_unlock(&lock) > > That's a deadlock even without copy_to_user - it's > enough for the thread to be preempted and another one > to try taking the lock. Huh? With CONFIG_PREEMPT spin_lock will disable preemption. (we had preempt = server anyway). But please: One step back. The problem is not the good path. The problem is that we lost a debugging aid for a known to be broken case. In other words: Our code had a bug. Older kernels detected that kind of bug. With your change we no longer saw the sleeping while atomic. Thats it. See my other mail. Christian