Message-ID: <450760.39028.qm@web53709.mail.re2.yahoo.com>
Date:	Wed, 23 Jan 2008 14:59:07 -0800 (PST)
From:	Nagendra Tomar <tomer_iisc@...oo.com>
To:	James.Smart@...lex.Com
Cc:	James.Bottomley@...elEye.com, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2.6.23.14] SCSI : scsi_device_lookup/scsi_device_lookup_by_target return NULL for an existent scsi_device

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.

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. 

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. 

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 fixed as a side effect of the patch.

Comments welcome.

Thanx,
Tomar





--- James Smart <James.Smart@...lex.Com> 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@...ptec.com>
> > ---
> > 
> > --- 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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> > -
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> > 
> 



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