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

Andrew Morton
2007-05-09T23:46:42Z

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

> From: Tim Hockin <thockin@google.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@google.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;
> +}