Commit f528e7ba authored Jul 21, 2007 by Tim Hockin Committed by Linus Torvalds Jul 21, 2007

x86_64: O_EXCL on /dev/mcelog



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.

Signed-off-by: Tim Hockin <thockin@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent a5ba7971

arch/x86_64/kernel/mce.c

+36 −0

Original line number	Diff line number	Diff line
		@@ -465,6 +465,40 @@ void __cpuinit mcheck_init(struct cpuinfo_x86 *c)
		* 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? */

		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;
		}

		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;
		}

		static void collect_tscs(void *data)
		{
		unsigned long cpu_tsc = (unsigned long )data;
		@@ -555,6 +589,8 @@ static int mce_ioctl(struct inode i, struct file f,unsigned int cmd, unsigned
		}

		static const struct file_operations mce_chrdev_ops = {
		.open = mce_open,
		.release = mce_release,
		.read = mce_read,
		.ioctl = mce_ioctl,
		};

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