linux-kernel - Re: [PATCH v8 01/10] kernfs: Remove reference counting for kernfs_open

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Date:   Tue, 26 Apr 2022 11:43:38 +1000
From:   Imran Khan <imran.f.khan@...cle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     viro@...iv.linux.org.uk, gregkh@...uxfoundation.org,
        ebiederm@...ssion.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 01/10] kernfs: Remove reference counting for
 kernfs_open_node.

Hello Tejun,

On 23/4/22 2:03 am, Tejun Heo wrote:
> On Sun, Apr 10, 2022 at 12:37:10PM +1000, Imran Khan wrote:
>> @@ -768,15 +765,15 @@ void kernfs_drain_open_files(struct kernfs_node *kn)
>>  	if (!(kn->flags & (KERNFS_HAS_MMAP | KERNFS_HAS_RELEASE)))
>>  		return;
>>  
>> -	spin_lock_irq(&kernfs_open_node_lock);
>>  	on = kn->attr.open;
>> -	if (on)
>> -		atomic_inc(&on->refcnt);
>> -	spin_unlock_irq(&kernfs_open_node_lock);
>>  	if (!on)
>>  		return;
>>
>>  	mutex_lock(&kernfs_open_file_mutex);
>> +	if (!kn->attr.open) {
>> +		mutex_unlock(&kernfs_open_file_mutex);
>> +		return;
>> +	}
> 
> What if @on got freed and new one got allocated between the lockless check
> and the locked check? Is there a reason to keep the lockless check at all?
> 

The only reason for lockless check is to opportunistically check and
return if ->attr.open is already NULL, without waiting to acquire the
mutex. This is because no one will be adding to ->attr.open at this
point of time.
But we can live with just the locked check as well.
Please let me know if you think of lockless check as an overkill in this
case.

Thanks
 -- Imran