linux-kernel - Re: [PATCH] x86_64: O_EXCL on /dev/mcelog (resend)

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Date:	Wed, 9 May 2007 16:45:55 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	thockin@...gle.com (Tim Hockin)
Cc:	ak@...e.de, vojtech@...e.cz, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86_64: O_EXCL on /dev/mcelog (resend)

On Mon, 7 May 2007 15:19:01 -0700
thockin@...gle.com (Tim Hockin) wrote:

> From: Tim Hockin <thockin@...gle.com>
> 
> Background:
>  /dev/mcelog is a clear-on-read interface.  It is currently possible for
>  multiple users to open and read() the device.  Users are protected from
>  each other during any one read, but not across reads.
> 
> Description:
>  This patch adds support for O_EXCL to /dev/mcelog.  If a user opens the
>  device with O_EXCL, no other user may open the device (EBUSY).  Likewise,
>  any user that tries to open the device with O_EXCL while another user has
>  the device will fail (EBUSY).
> 
> Result:
>  Applications can get exclusive access to /dev/mcelog.  Applications that
>  do not care will be unchanged.
> 
> Alternatives:
>  A simpler choice would be to only allow one open() at all, regardless of
>  O_EXCL.
> 
> Testing:
>  I wrote an application that opens /dev/mcelog with O_EXCL and observed
>  that any other app that tried to open /dev/mcelog would fail until the
>  exclusive app had closed the device.
> 
> Caveats:
>  None.
> 
> Patch:
>  This patch is against 2.6.21.
> 
> Signed-off-by: Tim Hockin <thockin@...gle.com>
> 
> ---
> 
> This is the first version version of this patch.  The simpler alternative
> of only one open() sounds better to me, but becomes a net change in
> behavior.
> 
> 
> diff -pruN linux-2.6.20+th/arch/x86_64/kernel/mce.c linux-2.6.20+th1.5/arch/x86_64/kernel/mce.c
> --- linux-2.6.20+th/arch/x86_64/kernel/mce.c	2007-04-27 14:19:08.000000000 -0700
> +++ linux-2.6.20+th1.5/arch/x86_64/kernel/mce.c	2007-05-01 21:53:10.000000000 -0700
> @@ -465,6 +465,40 @@ void __cpuinit mcheck_init(struct cpuinf
>   * Character device to read and clear the MCE log.
>   */
>  
> +static DEFINE_SPINLOCK(mce_state_lock);
> +static int open_count;	/* #times opened */
> +static int open_exclu;	/* already open exclusive? */

"open_exclusive" ;)

> +static int mce_open(struct inode *inode, struct file *file)
> +{
> +	spin_lock(&mce_state_lock);
> +
> +	if (open_exclu || (open_count && (file->f_flags & O_EXCL))) {
> +		spin_unlock(&mce_state_lock);
> +		return -EBUSY;
> +	}
> +
> +	if (file->f_flags & O_EXCL)
> +		open_exclu = 1;
> +	open_count++;
> +
> +	spin_unlock(&mce_state_lock);
> +
> +	return 0;
> +}

Does this guy support lseek?  If not we should toss a nonseekable_open()
call in here.

> +static int mce_release(struct inode *inode, struct file *file)
> +{
> +	spin_lock(&mce_state_lock);
> +
> +	open_count--;
> +	open_exclu = 0;
> +
> +	spin_unlock(&mce_state_lock);
> +
> +	return 0;
> +}

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