[PATCH] scsi: scsi_dh_alua: set handler_data before alua_initialize()

[Laurence Oberman <loberman@redhat.com>]

Since commit 50759b881e1d ("scsi: device_handler: alua: Call
scsi_device_put() from non-atomic context"), alua_rtpg_queue() only
queues an RTPG when sdev->handler_data points at an alua_dh_data whose
port group matches:

	struct alua_dh_data *h = sdev->handler_data;
	...
	if (h && rcu_dereference(h->pg) == pg) {

However alua_bus_attach() only assigns sdev->handler_data after
alua_initialize() returns, and alua_initialize() -> alua_check_vpd()
is where the initial RTPG is queued. At that point handler_data is
still NULL, so the initial RTPG is silently dropped: no RTPG is sent,
nothing is logged, and sdev->access_state keeps its zeroed default of
active/optimized for every path.

With dm-multipath this goes unnoticed, as pg_init calls
scsi_dh_activate() once the map is loaded, which queues an RTPG after
handler_data is set, and multipathd's alua prioritizer issues its own
RTPG from userspace. Anything relying on sdev->access_state without a
subsequent activate (sysfs access_state/preferred_path, native SCSI
multipath path selection) sees every path as active/optimized until a
Unit Attention or rescan triggers an RTPG.

Seen on an HP P2000 G3 FC array (implicit ALUA, two port groups, four
paths per LUN) with dm-multipath not in use: sg_rtpg reports port group
0 active/non-optimized and port group 1 active/optimized, but
/sys/class/scsi_device/H:C:T:L/device/access_state reads
active/optimized on all four paths after boot, and no
"alua: port group XX state ..." message is logged. Writing "activate"
to dh_state sends the RTPG and corrects access_state.

Kernels without commit 50759b881e1d queue the attach-time RTPG
unconditionally and are not affected.

Fix by assigning sdev->handler_data before calling alua_initialize(),
and clearing it again on failure. sdev->handler is only set once
scsi_dh_handler_attach() sees ->attach() succeed, so no handler
callback can observe the partially initialised alua_dh_data. On the
failure paths of alua_initialize(), h has not been added to a port
group and no RTPG has been queued, so freeing it is unchanged.

Fixes: 50759b881e1d ("scsi: device_handler: alua: Call scsi_device_put() from non-atomic context")
Cc: stable@vger.kernel.org
Signed-off-by: Laurence Oberman <loberman@redhat.com>
---

Notes:
    Tested on an HP P2000 G3 FC array (30 LUNs, 4 paths each): with this
    patch the "alua: port group 00 state N ..." messages are logged during
    boot and access_state shows two active/optimized and two
    active/non-optimized paths per LUN without any manual activate.
    Without it, all paths read active/optimized until an RTPG is triggered.
    
    I found this while testing John Garry's native SCSI multipath series
    (v9) on an HP P2000 G3 FC array (30 LUNs, 4 paths each, dm-multipath
    not in use). Native multipath uses sdev->access_state for path
    selection, and every path showed active/optimized, although sg_rtpg
    and dm-multipath (alua prio 50/10) both report port group 0 as
    active/non-optimized. No "alua: port group XX state ..." message was
    logged at boot. Writing "activate" to dh_state corrected access_state,
    which pointed at the attach-time RTPG never being queued.
    
    With this patch, the port group state messages are logged during boot
    and access_state shows two active/optimized and two
    active/non-optimized paths per LUN without any manual activate.
    
    This fix is independent of the native multipath series. It affects any
    user of access_state/preferred_path, and ALUA prep_fn fast-fail, on
    kernels with 50759b881e1d where nothing calls scsi_dh_activate()
    after attach.
    
    v9 series: https://lore.kernel.org/linux-scsi/20261108204733.557995-1-
    john.garry@linux.dev/

 drivers/scsi/device_handler/scsi_dh_alua.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/device_handler/scsi_dh_alua.c b/drivers/scsi/device_handler/scsi_dh_alua.c
index 87ac0a6d63e5..84ff3aed2c46 100644
--- a/drivers/scsi/device_handler/scsi_dh_alua.c
+++ b/drivers/scsi/device_handler/scsi_dh_alua.c
@@ -1233,13 +1233,20 @@ static int alua_bus_attach(struct scsi_device *sdev)
 	INIT_LIST_HEAD(&h->node);
 
 	mutex_init(&h->init_mutex);
+	/*
+	 * alua_rtpg_queue() only queues an RTPG when sdev->handler_data
+	 * points at this alua_dh_data, so set it before alua_initialize()
+	 * otherwise the initial RTPG issued from alua_check_vpd() is
+	 * silently dropped and sdev->access_state is never populated.
+	 */
+	sdev->handler_data = h;
 	err = alua_initialize(sdev, h);
 	if (err != SCSI_DH_OK && err != SCSI_DH_DEV_OFFLINED)
 		goto failed;
 
-	sdev->handler_data = h;
 	return SCSI_DH_OK;
 failed:
+	sdev->handler_data = NULL;
 	kfree(h);
 	return err;
 }
-- 
2.56.0