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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