linux-kernel - Re: [PATCH] use mutex instead of semaphore in tty

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Date:	Wed, 25 Apr 2007 20:13:59 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Matthias Kaehlcke <matthias.kaehlcke@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] use mutex instead of semaphore in tty_io.c

On Wed, Apr 25, 2007 at 05:49:34PM +0200, Matthias Kaehlcke wrote:
> drivers/char/tty_io.c uses a semaphore as mutex. use the mutex API
> instead of the (binary) semaphore

This looks like it should be a spinlock:

> -		down(&allocated_ptys_lock);
> +		mutex_lock(&allocated_ptys_lock);
>  		idr_remove(&allocated_ptys, idx);
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);

idr_remove is a quick operation that doesn't sleep.

> @@ -2639,24 +2639,24 @@ static int ptmx_open(struct inode * inode, struct file * filp)
>  	nonseekable_open(inode, filp);
>  
>  	/* find a device that is not in use. */
> -	down(&allocated_ptys_lock);
> +	mutex_lock(&allocated_ptys_lock);
>  	if (!idr_pre_get(&allocated_ptys, GFP_KERNEL)) {
> -		up(&allocated_ptys_lock);

The idr_pre_get should be moved out of the lock, that's the whole
point for it's existance..

> +		mutex_unlock(&allocated_ptys_lock);
>  		return -ENOMEM;
>  	}
>  	idr_ret = idr_get_new(&allocated_ptys, NULL, &index);
>  	if (idr_ret < 0) {
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);
>  		if (idr_ret == -EAGAIN)
>  			return -ENOMEM;
>  		return -EIO;
>  	}
>  	if (index >= pty_limit) {
>  		idr_remove(&allocated_ptys, index);
> -		up(&allocated_ptys_lock);
> +		mutex_unlock(&allocated_ptys_lock);
>  		return -EIO;
>  	}
> -	up(&allocated_ptys_lock);
> +	mutex_unlock(&allocated_ptys_lock);

And idr_get_new is another quick, non-blocking operation.

-
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