Bug 500729

The same workqueue is used both for sending uevents and processing queued I/O.
Deadlock has been reported in RHEL5 when sending a uevent was blocked waiting
for the queued I/O to be processed.  Use scheduled_work() for the asynchronous
uevents instead.

This is backport of fe9cf30eb8186ef267d1868dc9f12f2d0f40835a from 2.6.29-rc1.
Additional flush_scheduled_work() was added to avoid accessing freed memory
in case the device is destroyed before keventd dispatches the event (the same
fix for upstream is pending).

This may fix BZ 472790 as well. Awaiting test results. 

Signed-off-by: Mikulas Patocka <mpatocka>

diff -u -r -p linux-2.6.18.x86_64.p31/drivers/md/dm-mpath.c linux-2.6.18.x86_64/drivers/md/dm-mpath.c
--- linux-2.6.18.x86_64.p31/drivers/md/dm-mpath.c       2009-04-22 14:09:08.000000000 +0200
+++ linux-2.6.18.x86_64/drivers/md/dm-mpath.c   2009-05-08 14:33:58.000000000 +0200
@@ -856,6 +856,7 @@ static void multipath_dtr(struct dm_targ
 
        flush_workqueue(kmpath_handlerd);
        flush_workqueue(kmultipathd);
+       flush_scheduled_work();
        free_multipath(m);
 }
 
@@ -913,7 +914,7 @@ static int fail_path(struct pgpath *pgpa
        dm_path_uevent(DM_UEVENT_PATH_FAILED, m->ti,
                      pgpath->path.dev->name, m->nr_valid_paths);
 
-       queue_work(kmultipathd, &m->trigger_event);
+       schedule_work(&m->trigger_event);
 
 out:
        spin_unlock_irqrestore(&m->lock, flags);
@@ -955,7 +956,7 @@ static int reinstate_path(struct pgpath 
        dm_path_uevent(DM_UEVENT_PATH_REINSTATED, m->ti,
                      pgpath->path.dev->name, m->nr_valid_paths);
 
-       queue_work(kmultipathd, &m->trigger_event);
+       schedule_work(&m->trigger_event);
 
 out:
        spin_unlock_irqrestore(&m->lock, flags);
@@ -999,7 +1000,7 @@ static void bypass_pg(struct multipath *
 
        spin_unlock_irqrestore(&m->lock, flags);
 
-       queue_work(kmultipathd, &m->trigger_event);
+       schedule_work(&m->trigger_event);
 }
 
 /*
@@ -1029,7 +1030,7 @@ static int switch_pg_num(struct multipat
        }
        spin_unlock_irqrestore(&m->lock, flags);
 
-       queue_work(kmultipathd, &m->trigger_event);
+       schedule_work(&m->trigger_event);
        return 0;
 }