linux-kernel - Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver)

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Date:	Thu, 15 Nov 2012 20:22:27 +0100
From:	Herbert Poetzl <herbert@...hfloor.at>
To:	Paweł Sikora <pluto@...-linux.org>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	arekm@...-linux.org, baggins@...-linux.org,
	Daniel Hokka Zakrisson <daniel@...ac.com>
Subject: Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver)

On Thu, Nov 15, 2012 at 07:48:10PM +0100, Paweł Sikora wrote:
> On Tuesday 25 of September 2012 07:05:59 Herbert Poetzl wrote:
>> On Mon, Sep 24, 2012 at 11:17:42AM -0700, Eric W. Biederman wrote:
>>> Herbert Poetzl <herbert@...hfloor.at> writes:

>>>> On Mon, Sep 24, 2012 at 07:23:55AM +0200, Paweł Sikora wrote:
>>>>> On Sunday 23 of September 2012 18:10:30 Linus Torvalds wrote:
>>>>>> On Sat, Sep 22, 2012 at 11:09 PM, Paweł Sikora <pluto@...-linux.org> wrote:

>>>>>>>         br_read_lock(vfsmount_lock);

>>>>>> The vfsmount_lock is a "local-global" lock, where a read-lock
>>>>>> is rather cheap and takes just a per-cpu lock, but the
>>>>>> downside is that a write-lock is *very* expensive, and can
>>>>>> cause serious trouble.

>>>>>> And the write lock is taken by the [un]mount() paths. Do *not*
>>>>>> do crazy things. If you do some insane "unmount and remount
>>>>>> autofs" on a 1s granularity, you're doing insane things.

>>>>>> Why do you have that 1s timeout? Insane.

>>>>> 1s unmount timeout is *only* for fast bug reproduction (in few
>>>>> seconds after opteron startup) and testing potential patches.
>>>>> normally with 60s timeout it happens in few minutes..hours
>>>>> (depends on machine i/o+cpu load) and makes server unusable
>>>>> (permament soft-lockup).

>>>>> can we redesign vserver's mnt_is_reachable() for better locking
>>>>> to avoid total soft-lockup?

>>>> currently we do:

>>>>         br_read_lock(&vfsmount_lock);
>>>>         root = current->fs->root;
>>>>         root_mnt = real_mount(root.mnt);
>>>>         point = root.dentry;

>>>>         while ((mnt != mnt->mnt_parent) && (mnt != root_mnt)) {
>>>>                 point = mnt->mnt_mountpoint;
>>>>                 mnt = mnt->mnt_parent;
>>>>         }

>>>>         ret = (mnt == root_mnt) && is_subdir(point, root.dentry);
>>>>         br_read_unlock(&vfsmount_lock);

>>>> and we have been considering to move the br_read_unlock()
>>>> right before the is_subdir() call

>>>> if there are any suggestions how to achieve the same
>>>> with less locking I'm all ears ...

>>> Herbert, why do you need to filter the mounts that show up in a
>>> mount namespace at all?

>> that is actually a really good question!

>>> I would think a far more performant and simpler solution would
>>> be to just use mount namespaces without unwanted mounts.

>> we had this mechanism for many years, long before the
>> mount namespaces existed, and I vaguely remember that
>> early versions didn't get the proc entries right either

>> I took a quick look at the code and I think we can drop
>> the mnt_is_reachable() check and/or make it conditional
>> on setups without a mount namespace in place in the near
>> future (thanks for the input, really appreciated!)

> Hi,

> Herbert, can i just drop this mnt_is_reachable() method
> from vserver patch? this issue hasn't been solved for
> several months now. i can live without this problematic
> security-through-obscurity feature on my heavy loaded 
> machines.

sure, if you are aware of the implications, you can
simply remove the check ...

best,
Herbert

>>> I'd like to blame this on the silly rcu_barrier in
>>> deactivate_locked_super that should really be in the module
>>> remove path, but that happens after we drop the br_write_lock.

>>> The kernel take br_read_lock(&vfs_mount_lokck) during every rcu
>>> path lookup so mnt_is_reachable isn't particular crazy just for
>>> taking the lock.

>>> I am with Linus on this one. Paweł even 60s for your mount
>>> timeout looks too short for your workload. All of the readers
>>> that take br_read_lock(&vfsmount_lock) seem to be showing up in
>>> your oops. The only thing that seems to make sense is you have
>>> a lot of unmount activity running back to back, keeping the
>>> lock write held.

>>> The only other possible culprit I can see is that it looks like
>>> mnt_is_reachable changes reading /proc/mounts to be something
>>> worse than linear in the number of mounts and reading /proc/mounts
>>> starts taking the vfsmount_lock.  All minor things but when you
>>> are pushing things hard they look like things that would add up.

>>> Eric
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