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

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Date:	Mon, 24 Sep 2012 11:17:42 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Paweł Sikora <pluto@...-linux.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	arekm@...-linux.org, baggins@...-linux.org,
	Herbert Poetzl <herbert@...hfloor.at>
Subject: Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver)

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

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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