Re: Another idea for simplifying locking in kernel/module.c

[Adam J. Richter]

Rusty Russell wrote:
>In message <[e-mail]> you write:
>> Rusty Russell wrote:
>> >In message <[e-mail]> you write:
>> >> 	Here is a way to replace all of the specialized "stop CPU"
>> >> locking code in kernel/module.c with an rw_semaphore by using
>> >> down_read_trylock in try_module_get() and down_write when beginning to
>> >> unload the module.
>> 
>> >And now you can't modularize netfilter modules.
>> 
>> 	Why not?  Last time you went looking in the networking code
>> for an example of something that had to increment a module reference
>> in a context where blocking was not allowed you ended up conceding
>> that you example was incorrect.

>No, you're thinking of the IPv4 stack.  I didn't use netfilter as an
>example, because that opens me to "well, FIX NETFILTER then!".  If it
>were the only case, it's probably arguable.

>The problems with netfilter modules are exactly why I started looking
>at module locking over two years ago.

>> 	I just booted my gateway machine to a kernel using my
>> aforemetioned patch and various netfilter modules.  I've surfed the
>> web, FTP'ed file and run irc through it.  It seems to be okay.

>Sure!  That's because the netfilter modules use a horrific hack, by
>keeping their own "usage" counts and then spinning (potentially
>forever) on unload until it hits zero.
[...]

	Although I suspect that this could be fixed so that the
spinning is guanteed not to be forever, it happens that my
module_put(), which is the same as your module_put() is non-blocking
(as is my try_module_get(), by the way).  So I don't see what the
problem is.  You should still be able to call try_module_get and
module_put from an interrupt context.

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