linux-kernel - Re: [PATCH] cgroup/cpuset: remove unneeded nodes_or() in cpuset_change_task

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Message-ID: <c32a0bad-b20f-e53b-a38a-5da687b8a205@redhat.com>
Date:   Mon, 19 Jun 2023 10:37:57 -0400
From:   Waiman Long <longman@...hat.com>
To:     Miaohe Lin <linmiaohe@...wei.com>, tj@...nel.org,
        hannes@...xchg.org, lizefan.x@...edance.com
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cgroup/cpuset: remove unneeded nodes_or() in
 cpuset_change_task_nodemask()

On 6/17/23 04:30, Miaohe Lin wrote:
> The tsk->mems_allowed is changed before calling mpol_rebind_task() and
> being reassigned right after it. But tsk->mems_allowed is not needed
> inside mpol_rebind_task(). So remove unneeded tsk->mems_allowed modify
> via nodes_or() here.
>
> Signed-off-by: Miaohe Lin <linmiaohe@...wei.com>
> ---
>   kernel/cgroup/cpuset.c | 1 -
>   1 file changed, 1 deletion(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 58e6f18f01c1..33a429c1179f 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -1941,7 +1941,6 @@ static void cpuset_change_task_nodemask(struct task_struct *tsk,
>   	local_irq_disable();
>   	write_seqcount_begin(&tsk->mems_allowed_seq);
>   
> -	nodes_or(tsk->mems_allowed, tsk->mems_allowed, *newmems);
>   	mpol_rebind_task(tsk, newmems);
>   	tsk->mems_allowed = *newmems;
>   

That line was inserted by commit cc9a6c8776615 ("cpuset: mm: reduce 
large amounts of memory barrier related damage v3"). At first glance, it 
does looks like it is not necessary. However, I am not sure if a race is 
possible that will produce a false failure because of missing this line.

My 2 cents.

Cheers,
Longman