From 4dab46ff26c6003a13ec769312c50938b93c359d Mon Sep 17 00:00:00 2001
From: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Date: Thu, 22 Jul 2010 14:17:00 -0700
Subject: [PATCH] xen/blkfront: use tagged queuing for barriers

When barriers are supported, then use QUEUE_ORDERED_TAG to tell the block
subsystem that it doesn't need to do anything else with the barriers.
Previously we used ORDERED_DRAIN which caused the block subsystem to
drain all pending IO before submitting the barrier, which would be
very expensive.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
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