Re: [RESEND][PATCH] [SCSI] scsi_dh: allow 3rd party multipathdrivers to use scsi_dh_detach

From: Chandra Seetharaman
Date: Fri Apr 20 2012 - 11:50:33 EST


First off, sorry for not responding on this earlier. When I saw the
original patch, I was thinking it is a generic policy issue, and James
would be a better person to make decision on this, and forgot about it.

Now only realized that James was waiting on me.

I am not very religious either, and agree with James on changing the

With that change,

Acked-by: Chandra Seetharaman <sekharan@xxxxxxxxxx>


On Fri, 2012-04-20 at 19:17 +0400, James Bottomley wrote:
> On Fri, 2012-04-20 at 10:45 -0400, Mike Snitzer wrote:
> > Allow 3rd party multipath drivers to programatically detach a scsi_dh
> > using the scsi_dh_detach() interface. This is as improvement over
> > requiring them to write 'detach' to /sys/block/sdX/queue/dh_state
> >
> > End result is both Linux and 3rd party multipath drivers can coexist
> > without compromising Linux's default handling of multipath LUNs.
> >
> > Linux has suffered from races associated with attaching a scsi_dh to a
> > device too late (after an HBA driver has started the SCSI device scan).
> > Attaching a scsi_dh too late results in default sense handling that does
> > not silently fail IO to passive paths, which creates excessive delays
> > and IO errors during normal boot on a system with hundreds of LUNs.
> >
> > To fix this the appropriate scsi_dh must be attached before the HBA
> > driver(s) are even loaded. But some scsi_dh are known to conflict with
> > 3rd party multipath drivers (e.g. both scsi_dh_alua and scsi_dh_emc
> > conflict with EMC PowerPath). This patch allows 3rd party drivers to
> > resolve the conflict by detaching an attached scsi_dh.
> This changelog is a bit misleading, isn't it?
> The basic problem is that binary only third party drivers can't use the
> scsi_dh_detach callback because it's GPL only. The only module I know
> which suffers this problem is powerpath, is that right?
> So this patch actually relaxes our GPL only policy to allow powerpath to
> detach correctly. I really don't like the characterisation in the
> current proposed changelog of this being a "third party" problem because
> quite a few third party modules actually provide source and are thus GPL
> compliant.
> The whole point of GPL only symbols is to make life difficult for binary
> modules, so I could just say this might be indicative of the success of
> that policy. On the other hand, I'm not going to be religious about
> this. If you get the ack of the dh handler maintainer (Chandra
> Seetharaman) I'll apply this policy relaxation with a proper changelog
> that says what we're doing (allowing powerpath to bind to the symbol).
> James
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