Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752831AbaKZQvU (ORCPT ); Wed, 26 Nov 2014 11:51:20 -0500 Received: from e06smtp12.uk.ibm.com ([195.75.94.108]:52264 "EHLO e06smtp12.uk.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752764AbaKZQvS (ORCPT ); Wed, 26 Nov 2014 11:51:18 -0500 Message-ID: <547604FC.4030300@de.ibm.com> Date: Wed, 26 Nov 2014 17:51:08 +0100 From: Christian Borntraeger User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.2.0 MIME-Version: 1.0 To: "Michael S. Tsirkin" CC: David Hildenbrand , 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 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> <5475FAB1.1000802@de.ibm.com> <20141126163216.GB10850@redhat.com> In-Reply-To: <20141126163216.GB10850@redhat.com> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 14112616-0009-0000-0000-00000226DCFC Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Am 26.11.2014 um 17:32 schrieb Michael S. Tsirkin: [...] >>>> 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). > > Are you sure? Can you point me where it does this please? spin_lock --> raw_spin_lock --> _raw_spin_lock --> __raw_spin_lock static inline void __raw_spin_lock(raw_spinlock_t *lock) { ----> preempt_disable(); <----- spin_acquire(&lock->dep_map, 0, 0, _RET_IP_); LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock); } Michael, please be serious. The whole kernel would be broken if spin_lock would not disable preemption. > >> 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 > > You want to add more debugging tools, fine. We dont want to add, we want to fix something that used to work > But this one was > giving users in field false positives. So lets try to fix those, ok? If we cant, then tough luck. But coming up with wrong statements is not helpful. > > 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. 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. > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/