linux-kernel - Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able

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Date:   Tue, 10 Oct 2017 16:02:18 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Dennis Dalessandro <dennis.dalessandro@...el.com>,
        Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>
Cc:     bigeasy@...utronix.de, linux-rt-users@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Clark Williams <williams@...hat.com>,
        Dean Luick <dean.luick@...el.com>,
        Doug Ledford <dledford@...hat.com>,
        Kaike Wan <kaike.wan@...el.com>,
        Leon Romanovsky <leonro@...lanox.com>,
        linux-rdma@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Sebastian Sanchez <sebastian.sanchez@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] IB/hfi1: Use preempt_{dis,en}able_nort()

Em Tue, Oct 10, 2017 at 02:59:18PM -0400, Dennis Dalessandro escreveu:
> On 10/3/2017 11:49 AM, Arnaldo Carvalho de Melo wrote:
> > From: Arnaldo Carvalho de Melo <acme@...hat.com>
> > 
> > sc_buffer_alloc() disables preemption that will be reenabled by either
> > pio_copy() or seg_pio_copy_end(). But before disabling preemption it
> > grabs a spin lock that will be dropped after it disables preemption,
> > which ends up triggering a warning in migrate_disable() later on.
> > 
> >      spin_lock_irqsave(&sc->alloc_lock)
> >        migrate_disable() ++p->migrate_disable -> 2
> >      preempt_disable()
> >      spin_unlock_irqrestore(&sc->alloc_lock)
> >        migrate_enable() in_atomic(), so just returns, migrate_disable stays at 2
> >      spin_lock_irqsave(some other lock) -> b00m
> > 
> > And the WARN_ON code ends up tripping over this over and over in
> > log_store().
> > 
> > Sequence captured via ftrace_dump_on_oops + crash utility 'dmesg'
> > command.
> > 
> > [512258.613862] sm-3297 16 .....11 359465349134644: sc_buffer_alloc <-hfi1_verbs_send_pio
> > [512258.613876] sm-3297 16 .....11 359465349134719: migrate_disable <-sc_buffer_alloc
> > [512258.613890] sm-3297 16 .....12 359465349134798: rt_spin_lock <-sc_buffer_alloc
> > [512258.613903] sm-3297 16 ....112 359465349135481: rt_spin_unlock <-sc_buffer_alloc
> > [512258.613916] sm-3297 16 ....112 359465349135556: migrate_enable <-sc_buffer_alloc
> > [512258.613935] sm-3297 16 ....112 359465349135788: seg_pio_copy_start <-hfi1_verbs_send_pio
> > [512258.613954] sm-3297 16 ....112 359465349136273: update_sge <-hfi1_verbs_send_pio
> > [512258.613981] sm-3297 16 ....112 359465349136373: seg_pio_copy_mid <-hfi1_verbs_send_pio
> > [512258.613999] sm-3297 16 ....112 359465349136873: update_sge <-hfi1_verbs_send_pio
> > [512258.614017] sm-3297 16 ....112 359465349136956: seg_pio_copy_mid <-hfi1_verbs_send_pio
> > [512258.614035] sm-3297 16 ....112 359465349137221: seg_pio_copy_end <-hfi1_verbs_send_pio
> > [512258.614048] sm-3297 16 .....12 359465349137360: migrate_disable <-hfi1_verbs_send_pio
> > [512258.614065] sm-3297 16 .....12 359465349137476: warn_slowpath_null <-migrate_disable
> > [512258.614081] sm-3297 16 .....12 359465349137564: __warn <-warn_slowpath_null
> > [512258.614088] sm-3297 16 .....12 359465349137958: printk <-__warn
> > [512258.614096] sm-3297 16 .....12 359465349138055: vprintk_default <-printk
> > [512258.614104] sm-3297 16 .....12 359465349138144: vprintk_emit <-vprintk_default
> > [512258.614111] sm-3297 16 d....12 359465349138312: _raw_spin_lock <-vprintk_emit
> > [512258.614119] sm-3297 16 d...112 359465349138789: log_store <-vprintk_emit
> > [512258.614127] sm-3297 16 .....12 359465349139068: migrate_disable <-vprintk_emit
> > 
> > According to a discussion (see Link: below) on the linux-rt-users
> > mailing list, this locking is done for performance reasons, not for
> > correctness, so use the _nort() variants to avoid the above problem.
> > 
> > Suggested-by: Julia Cartwright <julia@...com>
> > Cc: Clark Williams <williams@...hat.com>
> > Cc: Dean Luick <dean.luick@...el.com>
> > Cc: Dennis Dalessandro <dennis.dalessandro@...el.com>
> > Cc: Doug Ledford <dledford@...hat.com>
> > Cc: Kaike Wan <kaike.wan@...el.com>
> > Cc: Leon Romanovsky <leonro@...lanox.com>
> > Cc: linux-rdma@...r.kernel.org
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Sebastian Andrzej Siewior <sebastian.siewior@...utronix.de>
> > Cc: Sebastian Sanchez <sebastian.sanchez@...el.com>
> > Cc: Steven Rostedt <rostedt@...dmis.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Link: http://lkml.kernel.org/r/20170926210045.GO29872@jcartwri.amer.corp.natinst.com
> > Signed-off-by: Arnaldo Carvalho de Melo <acme@...hat.com>
> 
> I assume you are not asking for Doug to pick this up for linux-rdma and that
> this more of an RFC sort of deal and the intended destination is the -rt

Right, and so far there were no strong objection for this one to be
merged on the -rt tree, Sebastian, can you do it please? Adding Dennis'
reviewed-by, one of maintainers for this driver, ok?

> tree? Anyway, for this patch:
> 
> Reviewed-by: Dennis Dalessandro <dennis.dalessandro@...el.com>