linux-kernel - Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <jhjmu51eq2k.mognet@arm.com>
Date:   Wed, 17 Jun 2020 15:15:31 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Scott Wood <swood@...hat.com>
Subject: Re: [PATCH] sched: __set_cpus_allowed_ptr(): Check cpus_mask, not cpus_ptr


On 17/06/20 13:17, Sebastian Andrzej Siewior wrote:
> From: Scott Wood <swood@...hat.com>
>
> This function is concerned with the long-term cpu mask, not the
> transitory mask the task might have while migrate disabled.  Before
> this patch, if a task was migrate disabled at the time
> __set_cpus_allowed_ptr() was called, and the new mask happened to be
> equal to the cpu that the task was running on, then the mask update
> would be lost.
>
> Signed-off-by: Scott Wood <swood@...hat.com>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  kernel/sched/core.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1637,7 +1637,7 @@ static int __set_cpus_allowed_ptr(struct
>               goto out;
>       }
>
> -	if (cpumask_equal(p->cpus_ptr, new_mask))
> +	if (cpumask_equal(&p->cpus_mask, new_mask))
>               goto out;
>
>       /*

Makes sense, but what about the rest of the checks? Further down there is

        /* Can the task run on the task's current CPU? If so, we're done */
        if (cpumask_test_cpu(task_cpu(p), new_mask))
                goto out;

If the task is currently migrate disabled and for some stupid reason it
gets affined elsewhere, we could try to move it out - which AFAICT we don't
want to do because migrate disabled. So I suppose you'd want an extra
bailout condition here when the task is migrate disabled.

ISTR in RT you do re-check the affinity and potentially move the task away
when re-enabling migration, so that should work out all fine.