[PATCH v2] [SCSI] scsi_dh: change scsi_dh_detach export to EXPORT_SYMBOL

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Allow a proprietary non-GPL multipath driver, like EMC PowerPath, to
detach a scsi_dh using scsi_dh_detach.  This is an improvement over
requiring them to write 'detach' to /sys/block/sdX/queue/dh_state

End result is Linux and PowerPath can coexist without compromising
Linux's default handling of multipath LUNs.

Linux has suffered from the race where a scsi_dh is not available to be
attached to a SCSI device until too late (after an HBA driver has
started the SCSI device scan that may attach a scsi_dh).  Attaching a
scsi_dh too late results in default sense handling that does not
silently end IO to passive paths, which creates excessive delays and IO
errors during normal boot of a system with hundreds of LUNs.

To fix this the appropriate scsi_dh must be available to be attached
before an HBA driver's SCSI device scan.  But once attached, both
scsi_dh_alua and scsi_dh_emc are known to conflict with PowerPath --
allowing PowerPath to use scsi_dh_detach resolves such conflicts.

Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
Acked-by: Hannes Reinecke <hare@xxxxxxx>
Acked-by: Chandra Seetharaman <sekharan@xxxxxxxxxx>
 drivers/scsi/device_handler/scsi_dh.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

v2: header tweaks, added Chandra's Acked-by
    p.s. James, if you still think I'm too indirect or long-winded
    please feel free to change the header accordingly.  Thanks!

diff --git a/drivers/scsi/device_handler/scsi_dh.c b/drivers/scsi/device_handler/scsi_dh.c
index 23149b9..a550de1 100644
--- a/drivers/scsi/device_handler/scsi_dh.c
+++ b/drivers/scsi/device_handler/scsi_dh.c
@@ -578,7 +578,7 @@ void scsi_dh_detach(struct request_queue *q)
 static struct notifier_block scsi_dh_nb = {
 	.notifier_call = scsi_dh_notifier
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