Messages in this thread | | | Date | Tue, 14 Jan 2014 18:14:46 +0100 | From | Nicolas Schichan <> | Subject | livelock during MD device open |
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Hi,
I have recently been trying to find the cause a livelock occurring during MD device open.
The livelock happens when a process tries to open an MD device for the first time and another opens the same MD device and sends an invalid ioctl:
Process 1 Process 2 --------- ---------
md_alloc() mddev_find() -> returns a new mddev with hold_active == UNTIL_IOCTL add_disk() -> sends KOBJ_ADD uevent
(sees KOBJ_ADD uevent for device) md_open() md_ioctl(INVALID_IOCTL) -> returns ENODEV and clears mddev->hold_active md_release() md_put() -> deletes the mddev as hold_active is 0
md_open() mddev_find() -> returns a newly allocated mddev with mddev->gendisk == NULL -> returns with ERESTARTSYS (kernel restarts the open syscall)
As to how to fix this, I see two possibilities:
- don't set hold_active to 0 if err is -ENODEV in the abort_unlock path in md_ioctl().
- check cmd parameter early in md_ioctl() and return -ENOTTY if the cmd parameter is not a valid MD ioctl.
Please advise on the preferred way to fix this, I'll be glad to send a patch for whatever is the preferred solution.
I have also a simple C program that I can send should you want to reproduce the issue.
Regards,
-- Nicolas Schichan Freebox SAS
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