From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753320AbbDGKtX (ORCPT ); Tue, 7 Apr 2015 06:49:23 -0400 Received: from mail-wg0-f52.google.com ([74.125.82.52]:35163 "EHLO mail-wg0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753139AbbDGKtU (ORCPT ); Tue, 7 Apr 2015 06:49:20 -0400 Message-ID: <1428403756.3152.56.camel@gmail.com> Subject: Re: [PATCH v2 1/2] rtmutex Real-Time Linux: Fixing kernel BUG at kernel/locking/rtmutex.c:997! From: Mike Galbraith To: Peter Zijlstra Cc: Steven Rostedt , Thavatchai Makphaibulchoke , linux-kernel@vger.kernel.org, mingo@redhat.com, tglx@linutronix.de, linux-rt-users@vger.kernel.org, Sebastian Andrzej Siewior Date: Tue, 07 Apr 2015 12:49:16 +0200 In-Reply-To: <20150407102912.GK23123@twins.programming.kicks-ass.net> References: <1424395866-81589-1-git-send-email-tmac@hp.com> <1428369962-74723-1-git-send-email-tmac@hp.com> <1428369962-74723-2-git-send-email-tmac@hp.com> <20150406215959.4e8ad37b@grimm.local.home> <1428383383.3152.5.camel@gmail.com> <20150407102912.GK23123@twins.programming.kicks-ass.net> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.16.0 Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2015-04-07 at 12:29 +0200, Peter Zijlstra wrote: > On Tue, Apr 07, 2015 at 07:09:43AM +0200, Mike Galbraith wrote: > > On Mon, 2015-04-06 at 21:59 -0400, Steven Rostedt wrote: > > > > > > We really should have a rt_spin_trylock_in_irq() and not have the > > > below if conditional. > > > > > > The paths that will be executed in hard irq context are static. > > > They > > > should be labeled as such. > > > > +/* > > + * Special purpose for locks taken in interrupt context: Take and > > hold > > + * ->wait_lock lest PI catching us with our fingers in the cookie > > jar. > > + * Do NOT abuse. > > + */ > > +int __lockfunc rt_spin_trylock_in_irq(spinlock_t *lock) > > +{ > > + struct task_struct *owner; > > + if (!raw_spin_trylock(&lock->lock.wait_lock)) > > + return 0; > > + owner = idle_task(raw_smp_processor_id()); > > + if (!(rt_mutex_cmpxchg(&lock->lock, NULL, owner))) { > > + raw_spin_unlock(&lock->lock.wait_lock); > > + return 0; > > + } > > + spin_acquire(&lock->dep_map, 0, 1, _RET_IP_); > > + return 1; > > +} > > + > > +/* ONLY for use with rt_spin_trylock_in_irq(), do NOT abuse. */ > > +void __lockfunc rt_spin_trylock_in_irq_unlock(spinlock_t *lock) > > +{ > > + struct task_struct *owner = > > idle_task(raw_smp_processor_id()); > > + /* NOTE: we always pass in '1' for nested, for simplicity > > */ > > + spin_release(&lock->dep_map, 1, _RET_IP_); > > + BUG_ON(!(rt_mutex_cmpxchg(&lock->lock, owner, NULL))); > > + raw_spin_unlock(&lock->lock.wait_lock); > > +} > > + > > Can someone explain this braindamage? You should _NOT_ take mutexes > in > hardirq context. No.. really? ;-) If you have a spot where it'd be nice to do that despite it being somewhat (koff).. discouraged shall we say, you have to do something funky. Thomas had a patch to not raise sirq unconditionally for -rt to let nohz_full work, but it needs a lock that's converted to an rtmutex in -rt, and which doesn't want to be un-converted. Ergo, get funky. > And if its an irq thread, then the irq thread _IS_ the right owner, > the > thread needs to be boosted by waiters. No irq thread. > The idle thread cannot ever be owner of a mutex, that's complete and > utter bullshit. Not if you want to hide current from the deadlock detector lest it get upset and make box go boom. -Mike