linux-kernel - Re: Union mount and lockdep design issues

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Message-ID: <87liw367uj.fsf@tucsk.pomaz.szeredi.hu>
Date:	Tue, 12 Jul 2011 13:45:08 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	Michal Suchanek <hramrach@...trum.cz>
Cc:	Ian Kent <ikent@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	David Howells <dhowells@...hat.com>,
	Ric Wheeler <rwheeler@...hat.com>,
	Alexander Viro <aviro@...hat.com>,
	Christoph Hellwig <hch@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, Jeff Moyer <jmoyer@...hat.com>
Subject: Re: Union mount and lockdep design issues

Michal Suchanek <hramrach@...trum.cz> writes:

> On 12 July 2011 10:30, Miklos Szeredi <miklos@...redi.hu> wrote:
>> Michal Suchanek <hramrach@...trum.cz> 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.  The lock orderings are:
>>
>> -> /overlay
>>  -> /lower1
>>  -> /tmpoverlay
>>    -> /lower2
>>    -> /upper
>> -> /overlay2
>>  -> /lower2
>>  -> /tmpoverlay2
>>    -> /lower1
>>    -> /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.  At 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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