Re: dm-mpath: do not change SCSI device handler

[Mike Snitzer <snitzer@redhat.com>]

On Thu, Apr 04 2013 at  8:55am -0400,
Mikulas Patocka <mpatocka@redhat.com> wrote:

> 
> 
> On Thu, 4 Apr 2013, Mike Snitzer wrote:
> 
> > I'll take a look at fixing this by deferring the scsi_dh switch until
> > resume.  This fix would assume multipath-tools is _not_ doing a noflush
> > suspend/resume when it is switching the scsi_dh.
> > 
> > Mike
> 
> This won't work because scsi_dh_attach allocates memory and you can't 
> allocate memory when something is suspended.

Ah yeah, scsi_dh->attach allocates memory for scsi_dh_data.  But
couldn't those scsi_dh_* attach allocations be switched from GFP_KERNEL
to GFP_NOIO?

> If we want to make it possible to change device handler, we need to add a 
> counter to "struct scsi_dh_data", increment the counter when the device 
> handler issues some request, decrement the counter when it finishes the 
> request (it may be tricky because you can't decrement it from the device 
> handler module because the module may be unloaded as soon as you decrement 
> it). Then, you wait until the counter is zero and detach the handler.

Noted, but I haven't given up hope on just deferring the scsi_dh switch
quite yet.

Mike