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

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Date:	Fri, 28 Sep 2012 11:02:53 -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 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?
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