[lustre-devel] [PATCH v2] lustre: mdc: fix possible deadlock in chlg_open()

NeilBrown neilb at suse.com
Tue Nov 6 16:33:25 PST 2018

On Tue, Nov 06 2018, quentin.bouget at cea.fr wrote:

> Le 06/11/2018 à 04:11, NeilBrown a écrit :
>> On Mon, Nov 05 2018, quentin.bouget at cea.fr wrote:
>>> Le 04/11/2018 à 22:34, James Simmons a écrit :
>>>>> Lockdep reports a possible deadlock between chlg_open() and
>>>>> mdc_changelog_cdev_init()
>>>>> mdc_changelog_cdev_init() takes chlg_registered_dev_lock and then
>>>>> calls misc_register() which takes misc_mtx.
>>>>> chlg_open() is called while misc_mtx is held, and tries to take
>>>>> chlg_registered_dev_lock.
>>>>> If these two functions race, a deadlock can occur as each thread will
>>>>> hold one of the locks while trying to take the other.
>>>>> chlg_open() does not need to take a lock.  It only uses the
>>>>> lock to stablize a list while looking for the matching
>>>>> chlg_registered_dev, and this can be found directly by examining
>>>>> file->private_data.
>>>>> So remove chlg_obd_get(), and use file->private_data to find the
>>>>> obd_device.
>>>>> Also ensure the device is fully initialized before calling
>>>>> misc_register().  This means setting up some list linkage before the
>>>>> call, and tearing it down if there is an error.
>>>> I have been testing this but I'm no HSM expert. I pushed this patch
>>>> to OpenSFS branch as well.
>>>> https://jira.whamcloud.com/browse/LU-11617
>>>> https://review.whamcloud.com/#/c/33572/
>>>> Reviewed-by: James Simmons <jsimmons at infradead.org>
>>> Reviewed-by: Quentin Bouget <quentin.bouget at cea.fr>
>> Thanks to you both for the review.
>> NeilBrown
> Wait! I just realised there might be another issue!
> I think there is now a race between chlg_open() and 
> mdc_changelog_cdev_finish().

Hmmm.. yes.  Also another deadlock possibility as the locks can be taken
in the wrong order here too.

> Wait! I just realised there might be another bigger issue!
> The whole "take the first obd you can find" is broken! I opened a ticket 
> <https://jira.whamcloud.com/browse/LU-11626> on whamcloud's JIRA about it.

My guess is that chlg_load(), chlg_clear() and (possibly)
chlg_read_cat_process_cb() should take the mutex, choose an obd, used
it, and release the mutex.

I might post an RFC patch.


> Quentin Bouget
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