Attempt to mount ext4 causes kernel BUG if incorrect mount option is used:

[root@rhel5 ~]# mount /tmp/ext4-image /mnt -t ext4 -o loop,blahblah
Segmentation fault
[root@rhel5 ~]#
Message from syslogd@ at Thu Mar 10 17:17:00 2011 ...
centos5 kernel: invalid opcode: 0000 [1] SMP 

I know ext4 code is in preview state, still would be nice to fix this.

The problem lies in fact that when s_blockgroup_lock separation code was merged in from .32 kernel, the separation was all thrown out, yet the freeing was left in place - patch linux-2.6-fs-ext4-~32-codebase.patch

So now if mount fails, this code in ext4_fill_super is executed:

 out_fail:                                                                      
        sb->s_fs_info = NULL;
+       kfree(&sbi->s_blockgroup_lock);   <==== Wrong!
        kfree(sbi);
        lock_kernel();
        return ret;

Since s_blockgroup_lock is defined as:
+       struct blockgroup_lock s_blockgroup_lock;                               


you can see that attempting to free it separately is a bad idea.