Re: Adding PCMCIA support to the kernel tree -- developers needed.

[Keith Owens <kaos@ocs.com.au>]

FWIW there has been some discussion between kernel developers about
redesigning the module load/unload mechanism.  Rusty Russell points out
that controlling module use counts by exporting the module structure
via the owner field does not work when the use count needs to be
manipulated from an interrupt.  In Rusty's case he is thinking about
netfilter, but the same point applies to hot unplugging.

Current thinking is to change modules so they have four required
functions, instead of the current two functions.

  init:		Allocate resources.
  register:	Make services available.
  unregister:	Remove services.
  exit:		Deallocate resources.

Module startup automatically calls init then register.  Conditional
shutdown checks for zero use count, calls unregister then flushes any
other cpus that had just entered the module.  If the flush was
successful, call exit.  If the flush was unsuccessful (somebody just
started using the module), call register to make the services available
again.

There is also an unconditional shutdown mode.  Call unregister and even
if the module is still in use, do not call register again.  When the
use count finally goes to zero, the exit routine is called and the
module is removed.

I think this will help hot unplugging.  When the device disappears, the
driver internally marks its hardware as unusable and schedules an
unconditional shutdown of itself, all out of the interrupt handler.

Unconditional shutdown calls unregister which stops any new code
entering the module.  The driver returns -EIO for all outstanding
requests which will eventually result in the outstanding users calling
close() or its equivalent and leaving the module.  When the use count
finally goes to zero, exit releases the resources and the module is
removed.  For built in code we do almost the same, the only difference
is that built in code is not removed from memory.


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