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From:   Keith Owens <kaos@ocs.com.au>
To:     Andrew Morton <andrewm@uow.edu.au>
cc:     "netdev@oss.sgi.com" <netdev@oss.sgi.com>
Subject: Re: modular net drivers, take 2 
In-reply-to: Your message of "Wed, 21 Jun 2000 02:37:58 GMT."
	     <39502A86.6C8C6FC2@uow.edu.au> 
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Date:   Wed, 21 Jun 2000 13:44:00 +1000
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On Wed, 21 Jun 2000 02:37:58 +0000, 
Andrew Morton <andrewm@uow.edu.au> wrote:
>Module unload is a very rare occurence, so let's penalise that and that
>alone.
>
>We grab the ENTIRE machine within sys_delete_module. 
>
>Like, grab the big kernel lock, then wait until ALL other CPUs are
>spinning on the kernel lock, and then allow sys_delete_module to
>proceed.

       CPU 0                              CPU 1
rmmod
use_count == 0, OK to remove
Grab big kernel lock.
Wait for other cpus to spin.
				  Enter open().
				  spin.
sys_delete_module runs.
module_exit() sleeps, drops lock.
				  open() continues, enters module.
				  MOD_INC_USE_COUNT
				  sleep in module open code, drop lock.
module_exit() wakes and continues.
Code is removed.
				  module open code wakes and continues.
				  Oops.

Anything sleeping loses the lock.  Any sleep in module open code primes
the race, if the module_exit code also sleeps the race is triggered.