From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <borntraeger@de.ibm.com>
Received: from e06smtp16.uk.ibm.com (e06smtp16.uk.ibm.com [195.75.94.112])
 (using TLSv1 with cipher DHE-RSA-AES256-SHA (256/256 bits))
 (No client certificate requested)
 by lists.ozlabs.org (Postfix) with ESMTPS id 4F1681A025A
 for <linuxppc-dev@lists.ozlabs.org>; Thu, 27 Nov 2014 02:30:45 +1100 (AEDT)
Received: from /spool/local
 by e06smtp16.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only!
 Violators will be prosecuted
 for <linuxppc-dev@lists.ozlabs.org> from <borntraeger@de.ibm.com>;
 Wed, 26 Nov 2014 15:30:38 -0000
Received: from b06cxnps4074.portsmouth.uk.ibm.com
 (d06relay11.portsmouth.uk.ibm.com [9.149.109.196])
 by d06dlp03.portsmouth.uk.ibm.com (Postfix) with ESMTP id C74261B08049
 for <linuxppc-dev@lists.ozlabs.org>; Wed, 26 Nov 2014 15:30:49 +0000 (GMT)
Received: from d06av09.portsmouth.uk.ibm.com (d06av09.portsmouth.uk.ibm.com
 [9.149.37.250])
 by b06cxnps4074.portsmouth.uk.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id
 sAQFUYfX10617180
 for <linuxppc-dev@lists.ozlabs.org>; Wed, 26 Nov 2014 15:30:34 GMT
Received: from d06av09.portsmouth.uk.ibm.com (localhost [127.0.0.1])
 by d06av09.portsmouth.uk.ibm.com (8.14.4/8.14.4/NCO v10.0 AVout) with ESMTP id
 sAQFUXis016765
 for <linuxppc-dev@lists.ozlabs.org>; Wed, 26 Nov 2014 08:30:34 -0700
Message-ID: <5475F218.4050207@de.ibm.com>
Date: Wed, 26 Nov 2014 16:30:32 +0100
From: Christian Borntraeger <borntraeger@de.ibm.com>
MIME-Version: 1.0
To: "Michael S. Tsirkin" <mst@redhat.com>,
 David Hildenbrand <dahi@linux.vnet.ibm.com>
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>
In-Reply-To: <20141126151729.GB9612@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 <linuxppc-dev.lists.ozlabs.org>
List-Unsubscribe: <https://lists.ozlabs.org/options/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=unsubscribe>
List-Archive: <http://lists.ozlabs.org/pipermail/linuxppc-dev/>
List-Post: <mailto:linuxppc-dev@lists.ozlabs.org>
List-Help: <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=help>
List-Subscribe: <https://lists.ozlabs.org/listinfo/linuxppc-dev>,
 <mailto:linuxppc-dev-request@lists.ozlabs.org?subject=subscribe>

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