Messages in this thread Patch in this message |  | | | Date | Mon, 7 May 2007 15:19:01 -0700 | | Subject | [PATCH] x86_64: O_EXCL on /dev/mcelog (resend) | | From | (Tim Hockin) |
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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? */ + +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; @@ -553,6 +587,8 @@ static int mce_ioctl(struct inode *i, st } static const struct file_operations mce_chrdev_ops = { + .open = mce_open, + .release = mce_release, .read = mce_read, .ioctl = mce_ioctl, }; - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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