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[Xen-devel] Re: analyze for the P1 bug 593(xensource bug tracker)

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Subject: [Xen-devel] Re: analyze for the P1 bug 593(xensource bug tracker)
From: han <vanbas.han@xxxxxxxxx>
Date: Wed, 10 May 2006 21:00:52 +0800
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Hi, Keir!

Your patch works quite well. We have created and destroyed the VMX more than 
500 times, and everything goes OK! I suppose the patch could solve the race 
condition! You may put the correctness code about VBD and VNIF together and 
send it to the maillist. We could help you to test it!
I prefer the wait_event and wakeup approach, it is clearer and straightful just 
as you said! :-)
BTW: I'm out of office right now, so i can't send the patch back to you! That's also why I change to another mailbox to send this mail.
Thanks a lot for your help!

Best Regards,

Han, Zhu д:
Best Regards, hanzhu

-----Original Message-----
From: Han, Zhu Sent: 2006$BG/(J5$B7n(J10$BF|(J 14:27
To: Yu, Ke; 'xen-devel@xxxxxxxxxxxxxxxxxxx'
Cc: Helix-vmm
Subject: analyze for the P1 bug 593(xensource bug tracker)

Hi, all!
Our QA team submitted a bug 593 to xensource bug tracker one month ago and it 
was boosted up to P1 several days ago! So I spend some time to trace this bug 
this week! Below words is what I have found:
1) This bug is hard to been reproduced on most of the platforms we owns, 
especially the UP box.  The platform on which we got the bug and could 
reproduce the bug stably is Paxville, which owns 4 physical CPUs, and 2 cores, 
2 hyperthreads for each CPU.
2) This root cause of this problem is "losetup -d /dev/loop*" could fail at a rather low 
probability. "losetup -d /dev/loop*" is invoked by /etc/xen/scripts/block when the script processes 
remove action. If we exhausted all the loop devices, the VMX cannot be initialized properly. That's why XEND 
complains "Error: Device creation failed for domain ****". However, if we remove the loop device 
manually, everything goes OK!
3) "losetup -d /dev/loop" failed because kernel/drivers/block/loop.c return 
EBUSY for the LOOP_CLR_FD ioctl operation. The probable cause for this action is some one 
else didn't close the loop device when we try to delete it!
4) The program opens the loop device could be VBD device driver. It opens the 
loop device in vbd_create() through open_by_devnum. It closes the handle for 
the loop device in vbd_free which is called by a schedulable work item 
free_blkif. Is it true? If so, the problem could be arised by the possible race 
condition between the work item and the hotplug script! When the xenbus driver 
is notified the front end device has been destroyed by the xenstore thread, it 
will remove the backend device and related resources, and then notify the 
hotplug subsystem the remove action! Because the code close the loop device's 
handle and the script delete the loop device can run concurrently, the script 
could fail when it try to delete the loop device!

My question is:
1) Does this possible race condition exist?
2) Why does the code closing the loop device been put to another out of code 
workitem instead of finishing all work directly in blkback_remove()? Any 
operation in free_blkif() could be blocked? Which one?

Since I'm a really newbie to this field, any tips and comments will be 
Thanks a lot!

Best Regards, hanzhu

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