linux-kernel - Re: [PATCH RT 21/23] sched: migrate_enable: Busy loop until the migration request is completed

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Date:   Tue, 03 Mar 2020 14:39:37 -0600
From:   Tom Zanussi <zanussi@...nel.org>
To:     Scott Wood <swood@...hat.com>, LKML <linux-kernel@...r.kernel.org>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Carsten Emde <C.Emde@...dl.org>,
        John Kacur <jkacur@...hat.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Daniel Wagner <wagi@...om.org>
Subject: Re: [PATCH RT 21/23] sched: migrate_enable: Busy loop until the
 migration request is completed

Hi Scott,

On Tue, 2020-03-03 at 13:56 -0600, Scott Wood wrote:
> On Thu, 2020-02-27 at 08:33 -0600, zanussi@...nel.org wrote:
> > From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > 
> > v4.14.170-rt75-rc2 stable review patch.
> > If anyone has any objections, please let me know.
> > 
> > -----------
> > 
> > 
> > [ Upstream commit 140d7f54a5fff02898d2ca9802b39548bf7455f1 ]
> > 
> > If user task changes the CPU affinity mask of a running task it
> > will
> > dispatch migration request if the current CPU is no longer allowed.
> > This
> > might happen shortly before a task enters a migrate_disable()
> > section.
> > Upon leaving the migrate_disable() section, the task will notice
> > that
> > the current CPU is no longer allowed and will will dispatch its own
> > migration request to move it off the current CPU.
> > While invoking __schedule() the first migration request will be
> > processed and the task returns on the "new" CPU with "arg.done =
> > 0". Its
> > own migration request will be processed shortly after and will
> > result in
> > memory corruption if the stack memory, designed for request, was
> > used
> > otherwise in the meantime.
> > 
> > Spin until the migration request has been processed if it was
> > accepted.
> > 
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Signed-off-by: Tom Zanussi <zanussi@...nel.org>
> > ---
> >  kernel/sched/core.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> As I said in https://marc.info/?l=linux-rt-users&m=158258256415340&w=
> 2 if
> you take thhis you should take the followup 2dcd94b443c5dcbc ("sched:
> migrate_enable: Use per-cpu cpu_stop_work")
> 

Yes, I didn't forget about this, it's just that I can't apply this to
4.14-rt until 4.19-rt does, otherwise it will be seen as a regression
to someone moving from 4.14-rt to 4.19-rt.

I will be keeping my eye out for when that happens and will apply it to
the next backport release at that point.

Thanks for making sure it wasn't missed in any case.

Tom 


> -Scott
> 
>