linux-kernel - VFS: Parallel unmounting of filesystems produces system call error

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Date:	Tue, 18 Nov 2014 16:49:35 +0100
From:	Philipp Hachtmann <phacht@...ux.vnet.ibm.com>
To:	viro@...iv.linux.org.uk
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: VFS: Parallel unmounting of filesystems produces system call error

Hi Al,

I just ran some tests involving many parallel mount and unmount
operations. I sometimes get EINVAL and the umount program says "not
mounted". Which is definitively wrong. It feels like a race condition
somewhere in the VFS code.

I use todays master branch (commit fc14f9c1272f62c3) on s390 with 1024
DASD devices.

In fs/namespace.c, sys_umount:

> 	retval = user_path_mountpoint_at(AT_FDCWD, name, 
> lookup_flags, &path);
...
> 	retval = -EINVAL;
> 	if (path.dentry != path.mnt->mnt_root)
> 		goto dput_and_out;
This seems to be erroneously true from time to time.

The script does something equivalent to:
> for n in $DEVNODES; do
>	umount $n &
> done

Do you have any idea what's wrong?

Kind regards

Philipp

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