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Commit f528e7ba authored by Tim Hockin's avatar Tim Hockin Committed by Linus Torvalds
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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: default avatarTim Hockin <thockin@google.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAndi Kleen <ak@suse.de>
Signed-off-by: default avatarLinus 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,

};