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

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Date:   Thu, 5 Mar 2020 13:38:48 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'zanussi@...nel.org'" <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

From: zanussi@...nel.org
> Sent: 27 February 2020 14:34
> [ 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.

What happens if the process changing the affinity mask is running
at a higher RT priority than that of the task being changed and
the new mask requires it run on the same cpu?

	David

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