block: fix __blkdev_get and add_disk race condition

The following situation might occur:

__blkdev_get:			add_disk:

				register_disk()
get_gendisk()

disk_block_events()
	disk->ev == NULL

				disk_add_events()

__disk_unblock_events()
	disk->ev != NULL
	--ev->block

Then we unblock events, when they are suppose to be blocked. This can
trigger events related block/genhd.c warnings, but also can crash in
sd_check_events() or other places.

I'm able to reproduce crashes with the following scripts (with
connected usb dongle as sdb disk).

<snip>
DEV=/dev/sdb
ENABLE=/sys/bus/usb/devices/1-2/bConfigurationValue

function stop_me()
{
	for i in `jobs -p` ; do kill $i 2> /dev/null ; done
	exit
}

trap stop_me SIGHUP SIGINT SIGTERM

for ((i = 0; i < 10; i++)) ; do
	while true; do fdisk -l $DEV  2>&1 > /dev/null ; done &
done

while true ; do
echo 1 > $ENABLE
sleep 1
echo 0 > $ENABLE
done
</snip>

I use the script to verify patch fixing oops in sd_revalidate_disk
http://marc.info/?l=linux-scsi&m=132935572512352&w=2


Without Jun'ichi Nomura patch titled "Fix NULL pointer dereference in
sd_revalidate_disk" or this one, script easily crash kernel within
a few seconds. With both patches applied I do not observe crash.
Unfortunately after some time (dozen of minutes), script will hung in:

[ 1563.906432]  [<c08354f5>] schedule_timeout_uninterruptible+0x15/0x20
[ 1563.906437]  [<c04532d5>] msleep+0x15/0x20
[ 1563.906443]  [<c05d60b2>] blk_drain_queue+0x32/0xd0
[ 1563.906447]  [<c05d6e00>] blk_cleanup_queue+0xd0/0x170
[ 1563.906454]  [<c06d278f>] scsi_free_queue+0x3f/0x60
[ 1563.906459]  [<c06d7e6e>] __scsi_remove_device+0x6e/0xb0
[ 1563.906463]  [<c06d4aff>] scsi_forget_host+0x4f/0x60
[ 1563.906468]  [<c06cd84a>] scsi_remove_host+0x5a/0xf0
[ 1563.906482]  [<f7f030fb>] quiesce_and_remove_host+0x5b/0xa0 [usb_storage]
[ 1563.906490]  [<f7f03203>] usb_stor_disconnect+0x13/0x20 [usb_storage]

Anyway I think this patch is some step forward.

As drawback, I do not teardown on sysfs file create error, because I do
not know how to nullify disk->ev (since it can be used). However add_disk
error handling practically does not exist too, and things will work
without this sysfs file, except events will not be exported to user
space.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Tejun Heo <tj@kernel.org>
Cc: stable@kernel.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>