From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751360AbeBTN5P (ORCPT ); Tue, 20 Feb 2018 08:57:15 -0500 Received: from merlin.infradead.org ([205.233.59.134]:56032 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751300AbeBTN5N (ORCPT ); Tue, 20 Feb 2018 08:57:13 -0500 Date: Tue, 20 Feb 2018 14:57:09 +0100 From: Peter Zijlstra To: christian.koenig@amd.com Cc: amd-gfx@lists.freedesktop.org, dri-devel@lists.freedesktop.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/4] locking/ww_mutex: add ww_mutex_is_owned_by function v3 Message-ID: <20180220135709.GD25201@hirez.programming.kicks-ass.net> References: <20180220125829.27060-1-christian.koenig@amd.com> <20180220131253.GF25314@hirez.programming.kicks-ass.net> <8fd80334-4d0e-8ed0-8a09-02a7e36a0eae@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <8fd80334-4d0e-8ed0-8a09-02a7e36a0eae@gmail.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Feb 20, 2018 at 02:26:55PM +0100, Christian König wrote: > > > +static inline bool ww_mutex_is_owned_by(struct ww_mutex *lock, > > > + struct ww_acquire_ctx *ctx) > > > +{ > > > + if (ctx) > > > + return likely(READ_ONCE(lock->ctx) == ctx); > > > + else > > > + return likely(__mutex_owner(&lock->base) == current); > > > +} > > Much better than the previous version. If you want to bike-shed, you can > > leave out the 'else' and unindent the last line. > > Thanks for the suggestion, going to do this. You might also want likely(ctx), since ww_mutex without ctx is a-typical I would think. > > I do worry about potential users of .ctx = NULL, though. It makes it far > > too easy to do recursive locking, which is something we should strongly > > discourage. > > Well, one of the addressed use cases is indeed checking for recursive > locking. But recursive locking is something rather normal for ww_mutex and > we are just exercising an existing code path. But that would be the ctx case, right? I'm not sure there is a lot of !ctx use out there, and in that case it really is rather like a normal mutex. > E.g. the most common use case for the ww_mutex is in the graphics drivers > where usespace sends us a list of buffer objects to work with. > > Now when userspace sends us duplicates in that buffer list the expectation > is to get -EALREADY from ww_mutex_lock when we try to lock the same ww_mutex > twice. Right, I remember that much.. :-) > The intention behind this function is now to a) be able to extend those > checks to make sure user space doesn't sends us potentially harmful nonsense > and b) allow to check for recursion in TTM during buffer object eviction > which uses ww_mutex_trylock instead of ww_mutex_lock. OK, but neither case would in fact need the !ctx case right? That's just there for completeness sake? But yes, I cannot think of a better fallback there either.