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

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Message-ID: <20120924112300.GE20655@MAIL.13thfloor.at>
Date:	Mon, 24 Sep 2012 13:23:00 +0200
From:	Herbert Poetzl <herbert@...hfloor.at>
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
Subject: Re: [2.6.38-3.x] [BUG] soft lockup - CPU#X stuck for 23s! (vfs, autofs, vserver)

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 ...

best,
Herbert

> BR,
> Paweł.

> ps).
> i'm adding Herbert to CC.
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