From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752145AbdAaN5C (ORCPT ); Tue, 31 Jan 2017 08:57:02 -0500 Received: from merlin.infradead.org ([205.233.59.134]:44540 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751955AbdAaN4u (ORCPT ); Tue, 31 Jan 2017 08:56:50 -0500 Date: Tue, 31 Jan 2017 12:25:26 +0100 From: Peter Zijlstra To: Jens Axboe Cc: "J. R. Okajima" , linux-kernel@vger.kernel.org, darrick.wong@oracle.com, david@fromorbit.com, longman@redhat.com, dave@stgolabs.net Subject: Re: Q: lockdep_assert_held_read() after downgrade_write() Message-ID: <20170131112526.GH6536@twins.programming.kicks-ass.net> References: <18295.1485811542@jrobl> <86195df9-2a43-2a0f-38ac-68792edc41c0@fb.com> <20170131103620.GM6515@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20170131103620.GM6515@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.23.1 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jan 31, 2017 at 11:36:20AM +0100, Peter Zijlstra wrote: > On Mon, Jan 30, 2017 at 02:30:45PM -0700, Jens Axboe wrote: > > I don't think you understand how it works. downgrade_write() turns a write > > lock into read held. To make that last sequence valid, you'd need: > > Correct, and I'm surprised that didn't explode in different ways. > > > > > down_write(&rw); > > downgrade_write(&rw); > > lockdep_assert_held_read(&rw) > > up_read(&rw); > > > > or just not drop up_write() from the last section. > > Right, but also, there seems to be a missing lockdep annotation to make > that work. That is, downgrade_write() doesn't have a lockdep annotation, > so it (lockdep) will still think its a write lock. > > > Let me try and fix both issues. Something like so I suppose,... completely untested. There could be a good reason for the current lockdep behaviour, but I cannot remember. --- diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index 45ba475d4be3..dfa9e40f83d5 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -123,10 +123,9 @@ EXPORT_SYMBOL(up_write); */ void downgrade_write(struct rw_semaphore *sem) { - /* - * lockdep: a downgraded write will live on as a write - * dependency. - */ + rwsem_release(&sem->dep_map, 1, _RET_IP_); + rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_); + rwsem_set_reader_owned(sem); __downgrade_write(sem); } diff --git a/kernel/locking/rwsem.h b/kernel/locking/rwsem.h index a699f4048ba1..3bd584c81b0b 100644 --- a/kernel/locking/rwsem.h +++ b/kernel/locking/rwsem.h @@ -40,8 +40,10 @@ static inline void rwsem_set_reader_owned(struct rw_semaphore *sem) * do a write to the rwsem cacheline when it is really necessary * to minimize cacheline contention. */ - if (sem->owner != RWSEM_READER_OWNED) + if (sem->owner != RWSEM_READER_OWNED) { + WARN_ON_ONCE(sem->owner != current); WRITE_ONCE(sem->owner, RWSEM_READER_OWNED); + } } static inline bool rwsem_owner_is_writer(struct task_struct *owner)