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 (file)
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2696,6 +2696,11 @@ static void flush_plug_list(struct blk_plug *plug)
 
        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);