linux-kernel - Re: [RESEND PATCH 5.10] fs/proc: do_task_stat: use __for_each

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Message-ID: <67083442-4717-4cd4-80a9-5790c535da99@oracle.com>
Date: Thu, 10 Jul 2025 19:29:31 +0530
From: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
To: Greg KH <gregkh@...uxfoundation.org>,
        "Heyne, Maximilian"
 <mheyne@...zon.de>
Cc: Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman"
 <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        Alexey Dobriyan <adobriyan@...il.com>,
        "Sauerwein, David"
 <dssauerw@...zon.de>,
        Sasha Levin <sashal@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [RESEND PATCH 5.10] fs/proc: do_task_stat: use
 __for_each_thread()

Hi Greg,

On 10/07/25 18:39, Greg KH wrote:
> On Thu, Jul 10, 2025 at 12:35:43PM +0000, Heyne, Maximilian wrote:
>> From: Oleg Nesterov <oleg@...hat.com>
>>
>> [ Upstream commit 7904e53ed5a20fc678c01d5d1b07ec486425bb6a ]
>>
>> do/while_each_thread should be avoided when possible.
>>
>> Link: https://lkml.kernel.org/r/20230909164501.GA11581@redhat.com
>> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
>> Cc: Eric W. Biederman <ebiederm@...ssion.com>
>> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
>> Stable-dep-of: 7601df8031fd ("fs/proc: do_task_stat: use sig->stats_lock to gather the threads/children stats")
>> Cc: stable@...r.kernel.org
>> [mheyne: adjusted context]
>> Signed-off-by: Maximilian Heyne <mheyne@...zon.de>
>> ---
>>
>> Compile-tested only.
>> We're seeing soft lock-ups with 5.10.237 because of the backport of
>> commit 4fe85bdaabd6 ("fs/proc: do_task_stat: use sig->stats_lock to
>> gather the threads/children stats").
> 
> And this fixes it?
> 

Our testing also showed that after the backport of this commit(on 5.15.y 
based release), we don't see the soft lockup anymore.

> How?

I think __for_each_thread() is safe whereas while_each_thread() is not safe.

This thread 
https://lore.kernel.org/all/20131202152437.GA10896@redhat.com/ explains 
why while_each_thread() is unsafe.

Thanks,
Harshit

> 
>>
>> ---
>>   fs/proc/array.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/fs/proc/array.c b/fs/proc/array.c
>> index 8fba6d39e776..77b94c04e4af 100644
>> --- a/fs/proc/array.c
>> +++ b/fs/proc/array.c
>> @@ -512,18 +512,18 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
>>   		cgtime = sig->cgtime;
>>   
>>   		if (whole) {
>> -			struct task_struct *t = task;
>> +			struct task_struct *t;
>>   
>>   			min_flt = sig->min_flt;
>>   			maj_flt = sig->maj_flt;
>>   			gtime = sig->gtime;
>>   
>>   			rcu_read_lock();
>> -			do {
>> +			__for_each_thread(sig, t) {
>>   				min_flt += t->min_flt;
>>   				maj_flt += t->maj_flt;
>>   				gtime += task_gtime(t);
>> -			} while_each_thread(task, t);
>> +			}
> 
> Ideally, the code generated here should be identical as before, so why
> is this change needed?
> > confused,>
> greg k-h