linux-kernel - Re: [PATCH] loop: Make explicit loop device destruction lazy

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Date:	Mon, 01 Oct 2012 10:00:00 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Dave Chinner <david@...morbit.com>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] loop: Make explicit loop device destruction lazy

Jens Axboe <axboe@...nel.dk> writes:

> On 2012-09-28 17:02, Jeff Moyer wrote:
>> Jens Axboe <axboe@...nel.dk> writes:
>> 
>>> On 2012-09-28 08:09, Dave Chinner wrote:
>>>> From: Dave Chinner <dchinner@...hat.com>
>>>>
>>>> xfstests has always had random failures of tests due to loop devices
>>>> failing to be torn down and hence leaving filesytems that cannot be
>>>> unmounted. This causes test runs to immediately stop.
>>>>
>>>> Over the past 6 or 7 years we've added hacks like explicit unmount
>>>> -d commands for loop mounts, losetup -d after unmount -d fails, etc,
>>>> but still the problems persist.  Recently, the frequency of loop
>>>> related failures increased again to the point that xfstests 259 will
>>>> reliably fail with a stray loop device that was not torn down.
>>>>
>>>> That is despite the fact the test is above as simple as it gets -
>>>> loop 5 or 6 times running mkfs.xfs with different paramters:
>>>>
>>>>         lofile=$(losetup -f)
>>>>         losetup $lofile "$testfile"
>>>>         "$MKFS_XFS_PROG" -b size=512 $lofile >/dev/null || echo "mkfs failed!"
>>>>         sync
>>>>         losetup -d $lofile
>>>>
>>>> And losteup -d $lofile is failing with EBUSY on 1-3 of these loops
>>>> every time the test is run.
>>>>
>>>> Turns out that blkid is running simultaneously with losetup -d, and
>>>> so it sees an elevated reference count and returns EBUSY.  But why
>>>> is blkid running? It's obvious, isn't it? udev has decided to try
>>>> and find out what is on the block device as a result of a creation
>>>> notification. And it is racing with mkfs, so might still be scanning
>>>> the device when mkfs finishes and we try to tear it down.
>>>>
>>>> So, make losetup -d force autoremove behaviour. That is, when the
>>>> last reference goes away, tear down the device. xfstests wants it
>>>> *gone*, not causing random teardown failures when we know that all
>>>> the operations the tests have specifically run on the device have
>>>> completed and are no longer referencing the loop device.
>>>
>>> I hear that %^#@#! blkid behavior, it is such a pain in the neck. I
>>> don't know how many times I've had to explain that behaviour to people
>>> who run write testing with tracing, wonder wtf there are reads in the
>>> trace.
>>>
>>> Patch looks fine, seems like the sane thing to do (lazy-remove on last
>>> drop) for this case.
>> 
>> Do we also want to prevent further opens?
>
> That's not a bad idea, at least it would be the logical thing to do. But
> it does get into the realm of potentially breaking existing behaviour.

What do you think could rely on the existing behaviour (that isn't
broken by design)?

-Jeff
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