linux-kernel - Re: [cgroup or VFS ?] WARNING: at fs/namespace.c:636 mntput_no

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Date:	Mon, 09 Feb 2009 16:49:40 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	LKML <linux-kernel@...r.kernel.org>,
	Paul Menage <menage@...gle.com>,
	Al Viro <viro@...IV.linux.org.uk>, containers@...ts.osdl.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [cgroup or VFS ?]  WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()

Andrew Morton wrote:
> (cc's added)
> 
> On Mon, 05 Jan 2009 11:23:33 +0800 Li Zefan <lizf@...fujitsu.com> wrote:
> 
>> Thread 1:
>>   for ((; ;))
>>   {
>>       mount -t cgroup -o cpuset xxx /mnt > /dev/null 2>&1
>>       mkdir /mnt/0 > /dev/null 2>&1
>>       rmdir /mnt/0 > /dev/null 2>&1
>>       umount /mnt > /dev/null 2>&1
>>   }
>>
>> Thread 2:
>>   for ((; ;))
>>   {
>>       mount -t cpuset xxx /mnt > /dev/null 2>&1
>>       umount /mnt > /dev/null 2>&1
>>   }
>>
>> (Note: Again it is irrelevant which cgroup subsys is used.)
>>
>> After a while this showed up:
>>
>> ------------[ cut here ]------------
>> WARNING: at fs/namespace.c:636 mntput_no_expire+0xac/0xf2()
>> Hardware name: Aspire SA85
>> Modules linked in: bridge stp llc autofs4 dm_mirror dm_region_hash dm_log dm_mod r8169 parport_pc mii parport sg button sata_sis pata_sis ata_generic libata sd_mod scsi_mod ext3 jbd mbcache uhci_hcd ohci_hcd ehci_hcd [last unloaded: scsi_wait_scan]
>> Pid: 4745, comm: umount Not tainted 2.6.28 #479
>> Call Trace:
>>  [<c042bbe3>] warn_slowpath+0x79/0x8f
>>  [<c044babf>] ? __lock_acquire+0x69a/0x700
>>  [<c04ae44e>] ? mntput_no_expire+0x79/0xf2
>>  [<c04ae481>] mntput_no_expire+0xac/0xf2
>>  [<c04ae968>] sys_umount+0x26a/0x2b1
>>  [<c04ae9c1>] sys_oldumount+0x12/0x14
>>  [<c0403251>] sysenter_do_call+0x12/0x31
>> ---[ end trace 79d0ab4bef01333f ]---
>>
>> The WARNING is: WARN_ON(atomic_read(&mnt->__mnt_writers));
> 
> OK, I'm all confused.  Here we see a WARN_ON triggered, but in
> http://lkml.org/lkml/2009/1/4/352 with the same testcase we're seeing a
> lockdep warning.
> 

They are 2 testcases with small difference ;)

case 1:
mount
cat whichever control file
umount

case 2:
mount
mkdir /cgroup/0
rmdir /cgroup/0
umount

> You refer to Arjan's "lockdep: annotate sb ->s_umount" patch - but
> that's over two years old.
> 
> And you say "The changelog said s_umount needs to be classified as
> per-sb, but actually it made it as per-filesystem." But what is the
> difference between per-sb and per-fs?
> 

a filesystem can be single-sb or multile, isn't it? that's struct super_lock
and struct file_system_type. I may be wrong here, since I don't know
much about VFS...

> More info here: http://bugzilla.kernel.org/show_bug.cgi?id=12673
> 
> This bug report seems to be all over the place.
> 
> Is it a post-2.6.28 regression, btw?
> 

I think it was introduced since cgroup was introduced. But it's hard to trigger
in real-life, though it's easy using this test case.


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