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

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Message-ID: <4DD3287A.2030808@jp.fujitsu.com>
Date:	Wed, 18 May 2011 11:01:30 +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

> diff --git a/fs/exec.c b/fs/exec.c
> index 5e62d26..34fa611 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -998,17 +998,28 @@ static void flush_old_files(struct files_struct * files)
> 
>   char *get_task_comm(char *buf, struct task_struct *tsk)
>   {
> -	/* buf must be at least sizeof(tsk->comm) in size */
> -	task_lock(tsk);
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&tsk->comm_lock, flags);
>   	strncpy(buf, tsk->comm, sizeof(tsk->comm));
> -	task_unlock(tsk);
> +	spin_unlock_irqrestore(&tsk->comm_lock, flags);
>   	return buf;
>   }
> 
>   void set_task_comm(struct task_struct *tsk, char *buf)
>   {
> +	unsigned long flags;
> +
> +	/*
> +	 * XXX - Even though comm is protected by comm_lock,
> +	 * we take the task_lock here to serialize against
> +	 * current users that directly access comm.
> +	 * Once those users are removed, we can drop the
> +	 * task locking&  memsetting.
> +	 */

If we provide __get_task_comm(), we can't remove memset() forever.


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

This is strange order. task_lock() doesn't disable interrupt.
And, can you please document why we need interrupt disabling?


>   	/*
>   	 * Threads may access current->comm without holding
>   	 * the task lock, so write the string carefully.


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