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 13:56:11 -0600
From:   Scott Wood <swood@...hat.com>
To:     zanussi@...nel.org, 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

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")

-Scott