[PATCH] blk_start_queue() must be called with irq disabled - add warning

The queue lock can be taken from interrupts so it must always be taken with
irq disabling primitives.  Some primitives already verify this.
blk_start_queue() is called under this lock, so interrupts must be
disabled.

Also document this requirement clearly in blk_init_queue(), where the queue
spinlock is set.
Signed-off-by: Paolo 'Blaisorblade' Giarrusso <blaisorblade@yahoo.it>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Jens Axboe <axboe@suse.de>

block/ll_rw_blk.c

@@ -1663,6 +1663,8 @@ static void blk_unplug_timeout(unsigned long data)
  **/
 void blk_start_queue(request_queue_t *q)
 {
+	WARN_ON(!irqs_disabled());
+
 	clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
 
 	/*
@@ -1878,7 +1880,8 @@ EXPORT_SYMBOL(blk_alloc_queue_node);
  *    get dealt with eventually.
  *
  *    The queue spin lock must be held while manipulating the requests on the
- *    request queue.
+ *    request queue; this lock will be taken also from interrupt context, so irq
+ *    disabling is needed for it.
  *
  *    Function returns a pointer to the initialized request queue, or NULL if
  *    it didn't succeed.