From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753009AbcGSHD0 (ORCPT ); Tue, 19 Jul 2016 03:03:26 -0400 Received: from mail-wm0-f67.google.com ([74.125.82.67]:33732 "EHLO mail-wm0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752661AbcGSHDW (ORCPT ); Tue, 19 Jul 2016 03:03:22 -0400 Date: Tue, 19 Jul 2016 09:03:17 +0200 From: Daniel Vetter To: Chris Wilson , Davidlohr Bueso , daniel.vetter@intel.com, jani.nikula@linux.intel.com, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [Intel-gfx] [rfc PATCH] drm/i915: Simplify shrinker_lock Message-ID: <20160719070317.GI17101@phenom.ffwll.local> Mail-Followup-To: Chris Wilson , Davidlohr Bueso , daniel.vetter@intel.com, jani.nikula@linux.intel.com, intel-gfx@lists.freedesktop.org, linux-kernel@vger.kernel.org References: <1468781144-31931-1-git-send-email-dave@stgolabs.net> <20160717215451.GB21839@nuc-i3427.alporthouse.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20160717215451.GB21839@nuc-i3427.alporthouse.com> X-Operating-System: Linux phenom 4.6.0-rc5+ User-Agent: Mutt/1.6.0 (2016-04-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jul 17, 2016 at 10:54:51PM +0100, Chris Wilson wrote: > On Sun, Jul 17, 2016 at 11:45:44AM -0700, Davidlohr Bueso wrote: > > In addition, we can simplify the overall function wrt (2), by first > > checking if we are the lock owner, then address the trylock and > > deal with (2) if locked/contended by a traditional mutex_lock(). > > This should be safe considering that if current is the lock owner, > > then we are guaranteed not to race with the counter->owner updates > > (the counter is updated first which sets the mutex to be visibly locked). > > However, that is then subject to an indirect ABBA deadlock, between the > shrinker lock and the struct mutex (or at least that used to be the case > where the kswapd reclaim would be blocked on the mutex and an alloc > blocked on kswapd). > > Unravelling the gross locking is an ongoing task, with one of the chief > goals being able to reclaim memory whenever required. It is not pretty > and often fails under pressure. Yeah, what we need is to split up the dev->struct_mutex Big Driver Lock to separate concerns. What's propably needed is a low-level mm lock (under which we never ever allocate anything to avoid the deadlock with reclaim). Plus probably per-object locks (using ww_mutex) to be able to protect buffer against both from the shrinker (which would trylock, considering locked objects busy) against threads and each another. We also might need per-submission context locks to avoid havoc there, but not sure. The reason this is taking forever to get done is that compared to the existing locking, this new scheme is even more complex ;-) -Daniel -- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch