On Tue, Apr 12, 2011 at 07:14:28AM +1000, NeilBrown wrote:

My understanding of that was that the calling requirement of
__elv_add_request is that the queue spinlock is held and that interrupts are
disabled.
So rather than possible enabling and disabling interrupts several times as
different queue are handled, the code just disabled interrupts once, and
then just take the spinlock once for each different queue.

The whole point of the change to plugging was to take locks less often.
Disabling interrupts less often is presumably an analogous goal.

Though I agree that a comment would help.

	q = NULL;
+	/* Disable interrupts just once rather than using spin_lock_irq/sin_unlock_irq
	 * variants
	 */
	local_irq_save(flags);


assuming my analysis is correct.

Your explanation does make sense to be now that you explain it.  I
didn't even thing of that variant before.