block: always allocate genhd->ev if check_events is implemented

commit 75e3f3e upstream.

9fd097b (block: unexport DISK_EVENT_MEDIA_CHANGE for legacy/fringe
drivers) removed DISK_EVENT_MEDIA_CHANGE from legacy/fringe block
drivers which have inadequate ->check_events().  Combined with earlier
change 7c88a16 (block: don't propagate unlisted DISK_EVENTs to
userland), this enables using ->check_events() for internal processing
while avoiding enabling in-kernel block event polling which can lead
to infinite event loop.

Unfortunately, this made many drivers including floppy without any bit
set in disk->events and ->async_events in which case disk_add_events()
simply skipped allocation of disk->ev, which disables whole event
handling.  As ->check_events() is still used during open processing
for revalidation, this can lead to open failure.

This patch always allocates disk->ev if ->check_events is implemented.
In the long term, it would make sense to simply include the event
structure inline into genhd as it's now used by virtually all block
devices.

Signed-off-by: Tejun Heo <[email protected]>
Reported-by: Ondrej Zary <[email protected]>
Reported-by: Alex Villacis Lasso <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>

block/genhd.c

@@ -1724,7 +1724,7 @@ static void disk_add_events(struct gendisk *disk)
 {
 	struct disk_events *ev;
 
-	if (!disk->fops->check_events || !(disk->events | disk->async_events))
+	if (!disk->fops->check_events)
 		return;
 
 	ev = kzalloc(sizeof(*ev), GFP_KERNEL);