block: add comment on why we save and disable interrupts in flush_plug_list()
It's done at the top to avoid doing it for every queue we unplug.
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/block/blk-core.c b/block/blk-core.c
index d20ce1e..0c0ea10 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2696,6 +2696,11 @@
q = NULL;
depth = 0;
+
+ /*
+ * Save and disable interrupts here, to avoid doing it for every
+ * queue lock we have to take.
+ */
local_irq_save(flags);
while (!list_empty(&list)) {
rq = list_entry_rq(list.next);