Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755314Ab2JCKxJ (ORCPT ); Wed, 3 Oct 2012 06:53:09 -0400 Received: from smtp-outbound-1.vmware.com ([208.91.2.12]:51027 "EHLO smtp-outbound-1.vmware.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750955Ab2JCKxH (ORCPT ); Wed, 3 Oct 2012 06:53:07 -0400 Message-ID: <506C190E.5050803@vmware.com> Date: Wed, 03 Oct 2012 12:53:02 +0200 From: Thomas Hellstrom User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120827 Thunderbird/15.0 MIME-Version: 1.0 To: Daniel Vetter CC: Maarten Lankhorst , linux-kernel@vger.kernel.org, dri-devel@lists.freedesktop.org, linaro-mm-sig@lists.linaro.org, sumit.semwal@linaro.org, linux-media@vger.kernel.org Subject: Re: [PATCH 1/5] dma-buf: remove fallback for !CONFIG_DMA_SHARED_BUFFER References: <20120928124148.14366.21063.stgit@patser.local> <5065B0C9.7040209@canonical.com> <5065FDAA.5080103@vmware.com> <50696699.7020009@canonical.com> <506A8DC8.5020706@vmware.com> <20121002080341.GA5679@phenom.ffwll.local> <506BED25.2060804@vmware.com> <506BF93B.5010805@vmware.com> In-Reply-To: Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2635 Lines: 61 On 10/03/2012 10:53 AM, Daniel Vetter wrote: > On Wed, Oct 3, 2012 at 10:37 AM, Thomas Hellstrom wrote: >>>> So if I understand you correctly, the reservation changes in TTM are >>>> motivated by the >>>> fact that otherwise, in the generic reservation code, lockdep can only be >>>> annotated for a trylock and not a waiting lock, when it *is* in fact a >>>> waiting lock. >>>> >>>> I'm completely unfamiliar with setting up lockdep annotations, but the >>>> only >>>> place a >>>> deadlock might occur is if the trylock fails and we do a >>>> wait_for_unreserve(). >>>> Isn't it possible to annotate the call to wait_for_unreserve() just like >>>> an >>>> interruptible waiting lock >>>> (that is always interrupted, but at least any deadlock will be catched?). >>> Hm, I have to admit that idea hasn't crossed my mind, but it's indeed >>> a hole in our current reservation lockdep annotations - since we're >>> blocking for the unreserve, other threads could potential block >>> waiting on us to release a lock we're holding already, resulting in a >>> deadlock. >>> >>> Since no other locking primitive that I know of has this >>> wait_for_unlocked interface, I don't know how we could map this in >>> lockdep. One idea is to grab the lock and release it again immediately >>> (only in the annotations, not the real lock ofc). But I need to check >>> the lockdep code to see whether that doesn't trip it up. >> >> I imagine doing the same as mutex_lock_interruptible() does in the >> interrupted path should work... > It simply calls the unlock lockdep annotation function if it breaks > out. So doing a lock/unlock cycle in wait_unreserve should do what we > want. > > And to properly annotate the ttm reserve paths we could just add an > unconditional wait_unreserve call at the beginning like you suggested > (maybe with #ifdef CONFIG_PROVE_LOCKING in case ppl freak out about > the added atomic read in the uncontended case). > -Daniel I think atomic_read()s are cheap, at least on intel as IIRC they don't require bus locking, still I think we should keep it within CONFIG_PROVE_LOCKING which btw reminds me there's an optimization that can be done in that one should really only call atomic_cmpxchg() if a preceding atomic_read() hints that it will succeed. Now, does this mean TTM can keep the atomic reserve <-> lru list removal? Thanks, Thomas -- 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/