From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christian Borntraeger Subject: Re: [RFC 0/2] Reenable might_sleep() checks for might_fault() when atomic Date: Wed, 26 Nov 2014 16:30:32 +0100 Message-ID: <5475F218.4050207@de.ibm.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> Mime-Version: 1.0 Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-path: Received: from e06smtp12.uk.ibm.com ([195.75.94.108]:55060 "EHLO e06smtp12.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752886AbaKZPai (ORCPT ); Wed, 26 Nov 2014 10:30:38 -0500 Received: from /spool/local by e06smtp12.uk.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Wed, 26 Nov 2014 15:30:37 -0000 In-Reply-To: <20141126151729.GB9612@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: "Michael S. Tsirkin" , David Hildenbrand Cc: linuxppc-dev@lists.ozlabs.org, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org, benh@kernel.crashing.org, paulus@samba.org, akpm@linux-foundation.org, heiko.carstens@de.ibm.com, schwidefsky@de.ibm.com, mingo@kernel.org 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