Re: Union mount and lockdep design issues

[Miklos Szeredi]

Michal Suchanek <hramrach@xxxxxxxxxx> writes:

> On 12 July 2011 10:30, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>> Michal Suchanek <hramrach@xxxxxxxxxx> writes:
>>
>>> The locking order is likely determined by the structure of the union
>>> and not some system-wide order of filesystems so assuming the readonly
>>> layers are locked as well you will probably get a deadlock with
>>> technically correct mount:
>>>
>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/upper /tmpoverlay
>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/tmpoverlay /overlay
>>>
>>> mount -t overlayfs overlayfs -olowerdir=/lower1,upperdir=/upper2 /tmpoverlay2
>>> mount -t overlayfs overlayfs -olowerdir=/lower2,upperdir=/tmpoverlay2 /overlay2
>>>
>>> because now lower1 and lower2 are differently ordered in the two
>>> overlays.
>>
>> Overlayfs never locks both upper and lower at the same time, which means
>> there's no AB-BA locking dependency. ÂThe lock orderings are:
>>
>> -> /overlay
>> Â-> /lower1
>> Â-> /tmpoverlay
>> Â Â-> /lower2
>> Â Â-> /upper
>> -> /overlay2
>> Â-> /lower2
>> Â-> /tmpoverlay2
>> Â Â-> /lower1
>> Â Â-> /upper2
>>
>> As you can see there's no nesting of lower2 and lower1 into each other.
>>
>> When you combine two filesystems, a completely new ordering is created
>> each time, there's no possibility to make an AB-BA nesting. ÂAt least I
>> cannot see one.
>
> Except you can get in situation where overlay locks lower1 and
> tmpoverlay waits for lower2

Note: tmpoverlay lock does *not* nest into lower1 lock, they are both on
the same nesting level.  There's no dependency between the two.

>  which is held by overlay2 waiting for
> lower1 in tmpoverlay2.

No deadlock there.

Thanks,
Miklos
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