[dm-devel] [RFC, PATCH] Proposed change to DM_DEV_WAIT implementation

[Kevin Corry]

A group here at IBM has been testing DM multipath and noticed a problem with
the DEVICE_WAIT command. In their tests, while performing a DEVICE_WAIT on a
multipath device, the command sometimes returns immediately, even though the
event-number is correct and no path-failure has occurred to trigger an event.
They tracked it down to the "waiting" method, and sent me this patch.

I remember some patches in -udm during the 2.6.4/2.6.5 time-frame that
switched some wake_up_interruptible()'s to wake_up()'s. I'm not sure if those
caused this problem, or if this is unrelated.

I'm also not certain if the change to mapped_device->event_nr and the
associated locking changes are the right way to go. We can discuss it and
see if this works, or if there is a better solution.

-Kevin

--- a/drivers/md/dm-ioctl.c	10 May 2004 09:20:57 -0000
+++ b/drivers/md/dm-ioctl.c	10 May 2004 17:37:45 -0000
@@ -850,7 +850,6 @@
 	int r;
 	struct mapped_device *md;
 	struct dm_table *table;
-	DECLARE_WAITQUEUE(wq, current);
 
 	md = find_device(param);
 	if (!md)
@@ -859,12 +858,10 @@
 	/*
 	 * Wait for a notification event
 	 */
-	set_current_state(TASK_INTERRUPTIBLE);
-	if (!dm_add_wait_queue(md, &wq, param->event_nr)) {
-		schedule();
-		dm_remove_wait_queue(md, &wq);
+	if (dm_wait_event(md, param->event_nr)) {
+		r = -ERESTARTSYS;
+		goto out;
 	}
- 	set_current_state(TASK_RUNNING);
 
 	/*
 	 * The userland program is going to want to know what
--- a/drivers/md/dm.c	10 May 2004 09:20:57 -0000
+++ b/drivers/md/dm.c	10 May 2004 17:37:45 -0000
@@ -80,7 +80,7 @@
 	/*
 	 * Event handling.
 	 */
-	uint32_t event_nr;
+	atomic_t event_nr;
 	wait_queue_head_t eventq;
 
 	/*
@@ -684,6 +684,7 @@
 	init_rwsem(&md->lock);
 	rwlock_init(&md->map_lock);
 	atomic_set(&md->holders, 1);
+	atomic_set(&md->event_nr, 0);
 
 	md->queue = blk_alloc_queue(GFP_KERNEL);
 	if (!md->queue)
@@ -753,10 +754,8 @@
 {
 	struct mapped_device *md = (struct mapped_device *) context;
 
-	down_write(&md->lock);
-	md->event_nr++;
+	atomic_inc(&md->event_nr);;
 	wake_up(&md->eventq);
-	up_write(&md->lock);
 }
 
 static void __set_size(struct gendisk *disk, sector_t size)
@@ -1054,35 +1053,13 @@
  *---------------------------------------------------------------*/
 uint32_t dm_get_event_nr(struct mapped_device *md)
 {
-	uint32_t r;
-
-	down_read(&md->lock);
-	r = md->event_nr;
-	up_read(&md->lock);
-
-	return r;
-}
-
-int dm_add_wait_queue(struct mapped_device *md, wait_queue_t *wq,
-		      uint32_t event_nr)
-{
-	down_write(&md->lock);
-	if (event_nr != md->event_nr) {
-		up_write(&md->lock);
-		return 1;
-	}
-
-	add_wait_queue(&md->eventq, wq);
-	up_write(&md->lock);
-
-	return 0;
+	return atomic_read(&md->event_nr);
 }
 
-void dm_remove_wait_queue(struct mapped_device *md, wait_queue_t *wq)
+int dm_wait_event(struct mapped_device *md, int event_nr)
 {
-	down_write(&md->lock);
-	remove_wait_queue(&md->eventq, wq);
-	up_write(&md->lock);
+	return wait_event_interruptible(md->eventq,
+			(event_nr != atomic_read(&md->event_nr)));
 }
 
 /*
--- a/drivers/md/dm.h	10 May 2004 09:20:57 -0000
+++ b/drivers/md/dm.h	10 May 2004 17:37:45 -0000
@@ -81,9 +81,7 @@
  * Event functions.
  */
 uint32_t dm_get_event_nr(struct mapped_device *md);
-int dm_add_wait_queue(struct mapped_device *md, wait_queue_t *wq,
-		      uint32_t event_nr);
-void dm_remove_wait_queue(struct mapped_device *md, wait_queue_t *wq);
+int dm_wait_event(struct mapped_device *md, int event_nr);
 
 /*
  * Info functions.