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Date:   Thu, 17 Mar 2022 11:24:31 +0100
From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     Zhang Qiao <zhangqiao22@huawei.com>
Cc:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>,
        linux-kernel@vger.kernel.org, stable@vger.kernel.org,
        Zhao Gongyi <zhaogongyi@huawei.com>,
        Waiman Long <longman@redhat.com>, Tejun Heo <tj@kernel.org>,
        Juri Lelli <juri.lelli@redhat.com>
Subject: Re: [PATCH 4.19 01/34] cgroup/cpuset: Fix a race between
 cpuset_attach() and cpu hotplug
Message-ID: <YjMMX7jSU8ynwgON@kroah.com>
References: <20220228172207.090703467@linuxfoundation.org>
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 <Yi73dKB10LBTGb+S@kroah.com>
 <aa25447a-f6ff-2ff2-72e9-3bbab1d430e9@huawei.com>
 <20220314111940.GC1035@blackbody.suse.cz>
 <YjHz2bifJBuCs/UK@kroah.com>
 <1ea13066-aa98-ead2-f50f-f62d030ce3c5@huawei.com>
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On Thu, Mar 17, 2022 at 10:41:57AM +0800, Zhang Qiao wrote:
> 
> 
> 在 2022/3/16 22:27, Greg Kroah-Hartman 写道:
> > On Mon, Mar 14, 2022 at 12:19:41PM +0100, Michal Koutný wrote:
> >> Hello.
> >>
> >> In my opinion there are two approaches:
> >> a) drop this backport (given other races present),
> > 
> > I have no problem with that, want to send a revert patch?
> > 
> >> b) swap the locks compatible with v4.19 as this patch proposes.
> >>
> >> On Mon, Mar 14, 2022 at 05:11:50PM +0800, Zhang Qiao <zhangqiao22@huawei.com> wrote:
> >>> +       /*
> >>> +        * It should hold cpus lock because a cpu offline event can
> >>> +        * cause set_cpus_allowed_ptr() failed.
> >>> +        */
> >>> +       cpus_read_lock();
> >>
> >> Maybe just a nit, the old kernels before commit c5c63b9a6a2e ("cgroup:
> >> Replace deprecated CPU-hotplug functions.") v5.15-rc1~159^2~5
> >> would be more consistent with get_online_cpus() here (but they're
> >> equivalent functionally so the locking order is correct).
> > 
> > A fixed up patch would also be appreciated :)
> > 
> 
> Fixed up patch as follows, replace cpus_read_lock() with get_online_cpus().
> 
> thanks.
> 
> --------
> 
> 
> [PATCH] cpuset: Fix unsafe lock order between cpuset lock and cpuslock
> 
> The backport commit 4eec5fe1c680a ("cgroup/cpuset: Fix a race
> between cpuset_attach() and cpu hotplug") looks suspicious since
> it comes before commit d74b27d63a8b ("cgroup/cpuset: Change
> cpuset_rwsem and hotplug lock order") v5.4-rc1~176^2~30 when
> the locking order was: cpuset lock, cpus lock.
> 
> Fix it with the correct locking order and reduce the cpus locking
> range because only set_cpus_allowed_ptr() needs the protection of
> cpus lock.
> 
> Fixes: 4eec5fe1c680a ("cgroup/cpuset: Fix a race between cpuset_attach() and cpu hotplug")
> Reported-by: Michal Koutný <mkoutny@suse.com>
> Signed-off-by: Zhang Qiao <zhangqiao22@huawei.com>
> ---
>  kernel/cgroup/cpuset.c | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index d43d25acc..4e1c4232e 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1528,9 +1528,13 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>         cgroup_taskset_first(tset, &css);
>         cs = css_cs(css);
> 
> -       cpus_read_lock();
>         mutex_lock(&cpuset_mutex);
> 
> +       /*
> +        * It should hold cpus lock because a cpu offline event can
> +        * cause set_cpus_allowed_ptr() failed.
> +        */
> +       get_online_cpus();
>         /* prepare for attach */
>         if (cs == &top_cpuset)
>                 cpumask_copy(cpus_attach, cpu_possible_mask);
> @@ -1549,6 +1553,7 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>                 cpuset_change_task_nodemask(task, &cpuset_attach_nodemask_to);
>                 cpuset_update_task_spread_flag(cs, task);
>         }
> +       put_online_cpus();
> 
>         /*
>          * Change mm for all threadgroup leaders. This is expensive and may
> @@ -1584,7 +1589,6 @@ static void cpuset_attach(struct cgroup_taskset *tset)
>                 wake_up(&cpuset_attach_wq);
> 
>         mutex_unlock(&cpuset_mutex);
> -       cpus_read_unlock();
>  }
> 
>  /* The various types of files and directories in a cpuset file system */
> --
> 2.18.0
> 
> 

Argh, whitespace was corrupted :(

I've fixed this up by hand and queued it up...

greg k-h