linux-kernel - Re: [PATCH 1/4] comm: Introduce comm_lock spinlock to protect task->comm access

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Message-ID: <4DD353E9.6020503@jp.fujitsu.com>
Date:	Wed, 18 May 2011 14:06:49 +0900
From:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
To:	john.stultz@...aro.org
CC:	linux-kernel@...r.kernel.org, joe@...ches.com, mingo@...e.hu,
	mina86@...a86.com, apw@...onical.com, jirislaby@...il.com,
	rientjes@...gle.com, dave@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, linux-mm@...ck.org
Subject: Re: [PATCH 1/4] comm: Introduce comm_lock spinlock to protect task->comm
 access

>> If we provide __get_task_comm(), we can't remove memset() forever.
>
> True enough. I'll fix that comment up then.
>
>>
>>>    	task_lock(tsk);
>>> +	spin_lock_irqsave(&tsk->comm_lock, flags);
>>
>> This is strange order. task_lock() doesn't disable interrupt.
>
> Strange order? Can you explain why you think that is? Having comm_lock
> as an inner-most lock seems quite reasonable, given the limited nature
> of what it protects.

spinlock -> irq_disable is wrong order.

local_irq_save()
task_lock()
spin_lock(task->comm)

is better. I think.

I mean if the task get interrupt at following point,

     	task_lock(tsk);
         // HERE
	spin_lock_irqsave(&tsk->comm_lock, flags);

the task hold task-lock long time rather than expected.

>> And, can you please document why we need interrupt disabling?
>
> Since we might access current->comm from irq context. Where would you
> like this documented? Just there in the code?

I'm prefer code comment. but another way is also good.


Thanks.

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