Description of problem:
Looking at the kernel printk output on shutdown of the kernel running in one of my virtual machines, I see a consistent 1 second delay after 'md: stopping all md devices' message is printed:

[    1.393524] md: stopping all md devices.
[    2.396385] ACPI: Preparing to enter system sleep state S5
[    2.397101] Disabling non-boot CPUs ...
[    2.397101] Power down.
[    2.397101] acpi_power_off called


Looking at the code in drivers/md/md.c, confirms there is an unconditional sleep in the shutdown path:


static int md_notify_reboot(struct notifier_block *this,
                            unsigned long code, void *x)
{
        struct list_head *tmp;
        mddev_t *mddev;

        if ((code == SYS_DOWN) || (code == SYS_HALT) || (code == SYS_POWER_OFF)) {

                printk(KERN_INFO "md: stopping all md devices.\n");

                for_each_mddev(mddev, tmp)
                        if (mddev_trylock(mddev)) {
                                /* Force a switch to readonly even array
                                 * appears to still be in use.  Hence
                                 * the '100'.
                                 */
                                md_set_readonly(mddev, 100);
                                mddev_unlock(mddev);
                        }
                /*
                 * certain more exotic SCSI devices are known to be
                 * volatile wrt too early system reboots. While the
                 * right place to handle this issue is the given
                 * driver, we do want to have a safe RAID driver ...
                 */
                mdelay(1000*1);
        }
        return NOTIFY_DONE;
}


I don't know whether the comment about exotic SCSI devices is still relevant, but at the very least this delay should be made conditional on the existence of any MD devices.

My virtual machine has no block devices, booting directly off a virtio 9p filesystem. From initial boot, through running the application, to shutting down, my virtual machine only runs for 1.9 seconds. 

In other words, this delay imposed by the MD driver is accounting for > 50% of my wallclock time, despite not having any MD devices in the guest.

Version-Release number of selected component (if applicable):
2.6.40.4-5.fc15.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Provision a (virtual) machine, which does *not* setup any MD devices.
1. Boot the (virtual) machine, ensuring the kernel log level is sufficient to show all printk lines
2. Shutdown the machine, watching the printk messages
3.
  
Actual results:
A 1 second delay occurs at the time this is printed:

[    1.393524] md: stopping all md devices.

Expected results:
There is no delay by the MD driver

Additional info: