Re: [PATCH] make the SCSI mid-layer obey the device online flag

[James Bottomley <James.Bottomley@steeleye.com>]

On Wed, 2003-06-04 at 12:51, Mike Anderson wrote:
> Doesn't this patch just re-implement in the prep_fn what is already
> being done by sd and sr in there init_command functions.

Yes, it's a precursor to consolidating them.

> Why allow io after online goes to zero. The user could bring the device
> back online if they needed to send IO. I was counting on no IO so we could
> do faster cleanup in the scsi_remove_host function.

Fundamentally, a queue is an asynchronous thing.  It is difficult (but
not imposible) to make the setting offline atomically guarantee no more
commands will be sent down.

However, on a philosophical level, it isn't necessarily desirable. 
Suppose we use offline to disconnect from a mounted filesystem (say
USB/Firewire unplug).  The user level might want to probe the device
before setting the online flag (which will resume the unerrored fs
transactions).

> I having been looking at this, but it is not very clean. To use existing
> common functionality and avoid deadlock from calling back into the
> request_fn the command needs wasted preparation just to use common
> interfaces.

Yes, that's why I think forbidding *all* I/O after offlining is too much
effort.  Offlining should be a precursor to device destruction, but
actual destruction probably relys on detaching the queue from the block
device interface and sitting on it until all use counts drop to zero.

James