What's the mean of FMODE_EXCL?

What's the mean of FMODE_EXCL?

[majianpeng]

Hi all:
	Today, i wrote to a partition which already mounted using dd.So the fs was destroyed completedly.
So i asked why dd can write? I remembed the flag O_EXCL which prevent other operation.
I review the code and found it used FMODE_EXCL.But why dd can write?
In open(2):
>             In  general,  the  behavior  of  O_EXCL is undefined if it is used without O_CREAT.  There is one
>             exception: on Linux 2.6 and later, O_EXCL can be used without O_CREAT if  pathname  refers  to  a
>              block device.  If the block device is in use by the system (e.g., mounted), open() fails with the
>              error EBUSY.
 But understand from the literal meaning, EXCL means opened exclusively.I think it like mutex-operation.
Can someone please tell me why?

Thanks!
Jianpeng