From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752031AbdFLMew (ORCPT ); Mon, 12 Jun 2017 08:34:52 -0400 Received: from merlin.infradead.org ([205.233.59.134]:46400 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751796AbdFLMeu (ORCPT ); Mon, 12 Jun 2017 08:34:50 -0400 Date: Mon, 12 Jun 2017 14:34:43 +0200 From: Peter Zijlstra To: "Levin, Alexander (Sasha Levin)" Cc: Linus Torvalds , Thomas Gleixner , Ingo Molnar , "linux-kernel@vger.kernel.org" Subject: Re: [PATCH] rt_mutex: correctly initialize lockdep in rt_mutex_init_proxy_locked Message-ID: <20170612123443.ddpij6zws6jydfdj@hirez.programming.kicks-ass.net> References: <20170610024705.13548-1-alexander.levin@verizon.com> <20170610140212.GM8337@worktop.programming.kicks-ass.net> <20170611145122.j3oxf6nsswc5okr7@sasha-lappy> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170611145122.j3oxf6nsswc5okr7@sasha-lappy> User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, Jun 11, 2017 at 02:51:09PM +0000, Levin, Alexander (Sasha Levin) wrote: > On Sat, Jun 10, 2017 at 04:02:12PM +0200, Peter Zijlstra wrote: > > On Sat, Jun 10, 2017 at 02:48:04AM +0000, Levin, Alexander (Sasha Levin) wrote: > > > lockdep can't deal with NULL name or key, and doesn't do anything > > > with the lock when that happens. > > > > Not doing anything is 'right', the proxy stuff won't be lockdep tracked > > anyway. But yeah, the first thing is a wee bit of a problem, for it will > > trigger DEBUG_LOCKS_WARN_ON() and fully kill lockdep. > > But don't we want pi_state->pi_mutex tracked by lockdep? Nope, we can't. That pi_mutex is owned by userspace and not all operations upon it are visible to the kernel. That is, a userspace thread can (conceptually) acquire the lock without the kernel ever knowing. We typically only create the pi_state when there's contention, at which point we create the pi_mutex as owned by someone else (hence proxy). Also, we 'obviously' hold the thing over the return to userspace, which is something lockdep very much doesn't like. > > Yeah, no need to do that; all we really need here is something like: > > > > --- > > kernel/locking/rtmutex-debug.c | 3 ++- > > 1 file changed, 2 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/locking/rtmutex-debug.c b/kernel/locking/rtmutex-debug.c > > index ac35e648b0e5..8dc647dc4b4b 100644 > > --- a/kernel/locking/rtmutex-debug.c > > +++ b/kernel/locking/rtmutex-debug.c > > @@ -175,7 +175,8 @@ void debug_rt_mutex_init(struct rt_mutex *lock, const char *name, struct lock_cl > > lock->name = name; > > > > #ifdef CONFIG_DEBUG_LOCK_ALLOC > > - lockdep_init_map(&lock->dep_map, name, key, 0); > > + if (name && key) > > + lockdep_init_map(&lock->dep_map, name, key, 0); > > #endif > > } > > I didn't want to do that because in later calls on that mutex we > will end up going into lockdep code, and I didn't think that doing > that without calling lockdep_init_map() initially was safe. So futex has its own private rt_mutex implementation and interface, none of which include lockdep hooks. So not initializing the field for that case should not be a problem.