[patch]block: document blk_plug list access

[Shaohua Li <shaohua.li@intel.com>]

I'm often confused why not disable preempt when changing blk_plug list. It
would be better to add comments here in case others have the similar concerns.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
 include/linux/blkdev.h |    6 ++++++
 1 file changed, 6 insertions(+)

Index: linux/include/linux/blkdev.h
===================================================================
--- linux.orig/include/linux/blkdev.h	2011-07-08 09:21:19.000000000 +0800
+++ linux/include/linux/blkdev.h	2011-07-08 09:34:39.000000000 +0800
@@ -857,6 +857,12 @@ struct request_queue *blk_alloc_queue(gf
 struct request_queue *blk_alloc_queue_node(gfp_t, int);
 extern void blk_put_queue(struct request_queue *);
 
+/*
+ * Note: Code in between changing the blk_plug list/cb_list or element of such
+ * lists is preemptable, but such code can't do sleep (or be very careful),
+ * otherwise data is corrupted. For details, please check schedule() where
+ * blk_schedule_flush_plug() is called.
+ */
 struct blk_plug {
 	unsigned long magic;
 	struct list_head list;