[dm-devel] dm-mpath: do not change SCSI device handler

[Mikulas Patocka]

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.

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.

Mikulas