Re: [PATCH 2.6.23.14] SCSI : scsi_device_lookup/scsi_device_lookup_by_targetreturn NULL for an existent scsi_device

From: James Smart
Date: Wed Jan 30 2008 - 16:34:23 EST

Nagendra Tomar wrote:
Hello James,
My understanding is that the scsi_device in SDEV_DEL state
is there in the scsi_host->devices/scsi_target->devices queue, just
because there is some outstanding command holding a reference to it.

Well, there's a lot more reasons than just an outstanding command. The
biggest issue is when there has been a downstream reference to it - such
as by a filesystem or DM object.

Some of the headaches have been these downstream references that are never
released. Ex: DM does not expect devices to come and go yet.

It will need the device when it completes. Apart from this, for all practical purposes the scsi_device is gone from the system. It could
as well be removed from scsi_host->devices/scsi_target->devices lists
and be put in some other list, just to hold the scsi_device till
commands refering to it are completed.

Keep thinking about DM references. So... it isn't quite gone from the
system.

The scanning code should not consider these devices to be present in the system. This is correctly handled today, as scsi_device_lookup/scsi_device_lookup_by_target return NULL if there
is an SDEV_DEL device in the list.

Since a scsi_device in SDEV_DEL state is "gone" from the system we should not hold any fresh references on to this device. The current
scsi_device_lookup/scsi_device_lookup_by_target implementation rightly
ensures that. And since all the sysfs linkages are also removed (in
__scsi_remove_device->device_del), user space can also not reference this device. All is good till now.

The problem happens when we try to add a new scsi_device with the same HBTL. Since we consider the device "gone" (as described above) we should
allow a new scsi_device with the same HBTL to be added (to the scsi_host->devices/scsi_target->devices list). This part is also correctly
implemented.
scsi_add_device->...->scsi_probe_and_add_lun->scsi_device_lookup_by_target
will _not_ return the device present in the SDEV_DEL state and hence the
scanning will go ahead and try to add a new scsi_device (this one in SDEV_RUNNING state) to the devices lists.

All is good even till now.

The PROBLEM is, now any scsi_device_lookup call trying to lookup this newly
added scsi_device, fails. This is because, scsi_device_lookup->__scsi_device_lookup
returns the first device that it finds in the list, which in this case
is the one in the SDEV_DEL state. Now the scsi_device_get call that scsi_device_lookup makes to get a reference on that device returns ENXIO
as the device is in SDEV_DEL state, resulting in scsi_device_lookup to return NULL.

What scsi_device_get does, is right, as we do not want to hold fresh references on scsi_devices in SDEV_DEL state. The problem is because
of this we fail to lookup the perfectly legitimate device (in SDEV_RUNNING
state) with the same HBTL sitting in the list.

One of the side effects of this is that the scsi_probe_and_add_lun()
goes ahead with the scanning and tries to add this existent device. This is the real problem.

My patch avoids this problem by not breaking from the __scsi_device_lookup
loop, if the device is in SDEV_DEL state. After all we should not consider
these devices to be part of the system. This will allow us to find the right scsi_device and this "trying to add an existent device"
problem will be avoided.

So nothing you've described so far, including your patch, is different
from the discussion in this thread: http://marc.info/?l=linux-scsi&m=115582805811406&w=2
which terminates with a recommendation from James B to avoid this kind of
patch as there's also a lurking issue with the SDEV_CANCEL state.

Given that since this thread:
- the consensus was to resurrect the sdev from the SDEV_DEL state back
to SDEV_RUNNING
http://marc.info/?l=linux-scsi&m=117139230702785&w=2
- that patches for the resurrection where implemented post this thread
and are in the upstream kernel
http://marc.info/?l=linux-scsi&m=117991046126294&w=2
- and given there is still a set of resurrection fixes outstanding
http://marc.info/?l=linux-scsi&m=118215727101887&w=2

I don't see any future for your patch.

Please apply the oustanding resurrection patch set and see if the proper
behavior occurs. If it doesn't, please post to l-s again and I'm sure
Hannes and I can consider it further.


And also why should scsi_device_lookup and __scsi_device_lookup be different in behaviour. One returns devices in SDEV_DEL state, the other
doesn't. The comments suggest that they can be used interchangibly, but
for the locking and the extra reference that the scsi_device_lookup holds.

This is somewhat of a different argument. We should answer/solve the resurrection
problem, then see where things lead us.

-- james s


This is fixed as a side effect of the patch.

Comments welcome.

Thanx,
Tomar





--- James Smart <James.Smart@xxxxxxxxxx> wrote:

This sounds like a return to the old behavior, where sdevs in SDEV_DEL
were ignored. However, it too had lots of bad effects. We'd have to go
back to the threads over the last 2 years that justified resurrecting
the sdev. Start looking at threads like :
http://marc.info/?l=linux-scsi&m=115555788730468&w=2
http://marc.info/?l=linux-scsi&m=116837744314913&w=2
http://marc.info/?l=linux-scsi&m=117139230702785&w=2
http://marc.info/?l=linux-scsi&m=117991046126294&w=2
Also, there's multiple parts to this - the sdev struct, and the sysfs objects
and thus namespace associated with the struct, etc.

So, in my mind, if this reverts to ignoring sdevs in SDEV_DEL, and creates
a duplicate sdev in SDEV_RUNNING, then it's the wrong patch. What should
be considered is where did the resurrection of the sdev go wrong. I
remember that Hannes did some updates
http://marc.info/?l=linux-scsi&m=118215727101887&w=2
but I don't believe these ever got merged upstream. Perhaps that's a good
place to start.

-- james s


Nagendra Tomar wrote:
__scsi_device_lookup and __scsi_device_lookup_by_target do not check for the sdev_state and hence return scsi_devices with sdev_state set to SDEV_DEL also. It has the following side effects.

We can have two scsi_devices with the same HBTL queued in the scsi_host->__devices/scsi_target->devices list, one
in the SDEV_DEL state and the other in, say SDEV_RUNNING state. If the one in the SDEV_DEL state is before the one in SDEV_RUNNING state, (which will almost always be the case) the scsi_device_lookup and scsi_device_lookup_by_target will never find the totally legitimate
scsi_device (the one in the SDEV_RUNNING state).

This is because __scsi_device_lookup/__scsi_device_lookup_by_target always returns the first one in the list (which in our case is the one with the SDEV_DEL state) and the scsi_device_get() which is called by scsi_device_lookup/scsi_device_lookup_by_target will return -ENXIO for this scsi_device, resulting in scsi_device_lookup and scsi_device_lookup_by_target to return NULL.

So we _cannot_ lookup a perfectly valid device present in the
list of scsi_devices.

The right thing to do is to not have __scsi_device_lookup
and __scsi_device_lookup_by_target match a device if the scsi_device
state is SDEV_DEL. This will also make these functions similar in behaviour to their scsi_device_lookup/scsi_device_lookup_by_target
counterparts, as the comments in the code suggest.

One way by which we can have two scsi_devices in the list is as follows. Suppose a scsi_device has some outstanding command(s) when scsi_remove_device is called for it. Due to the extra ref being held
by the command in flight, the __scsi_remove_device->put_device call will not actually free the scsi_device and it will remain in the scsi_device list albeit in the SDEV_DEL state. Now if we do a scsi_add_device for the same HBTL, a new device with the same HBTL
(this one in SDEV_RUNNING state) gets added to the scsi_device list. Infact if we call scsi_add_device one more time, it happily goes ahead and tries to add it once more, as scsi_probe_and_add_lun->scsi_device_lookup_by_target does not return
the already existing device. This will though result in the kobject EEXIST warning dump.

The patch below solves the problem described here by not
returning scsi_devices in SDEV_DEL state, thus allowing scsi_device
in SDEV_RUNNING state (if any) to be correctly returned, instead.


Thanx,
Tomar


Signed-off-by: Nagendra Singh Tomar <nagendra_tomar@xxxxxxxxxxx>
---

--- linux-2.6.23.14/drivers/scsi/scsi.c.orig 2008-01-23 18:06:02.000000000 +0530
+++ linux-2.6.23.14/drivers/scsi/scsi.c 2008-01-23 19:17:35.000000000 +0530
@@ -951,7 +951,7 @@ struct scsi_device *__scsi_device_lookup
struct scsi_device *sdev;
list_for_each_entry(sdev, &starget->devices, same_target_siblings) {
- if (sdev->lun ==lun)
+ if (sdev->lun == lun && sdev->sdev_state != SDEV_DEL)
return sdev;
}
@@ -1008,7 +1008,7 @@ struct scsi_device *__scsi_device_lookup
list_for_each_entry(sdev, &shost->__devices, siblings) {
if (sdev->channel == channel && sdev->id == id &&
- sdev->lun ==lun)
+ sdev->lun == lun && sdev->sdev_state != SDEV_DEL)
return sdev;
}



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