linux-kernel - Re: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module libcrc32c"

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Date:	Tue, 1 Jun 2010 10:51:24 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Brandon Philips <brandon@...p.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	LKML <linux-kernel@...r.kernel.org>,
	Jon Masters <jonathan@...masters.org>,
	Tejun Heo <htejun@...il.com>,
	Masami Hiramatsu <mhiramat@...hat.com>,
	Kay Sievers <kay.sievers@...y.org>
Subject: Re: [PATCH 2/2] module: fix bne2 "gave up waiting for init of module libcrc32c"

On Tue, 1 Jun 2010 03:49:14 am Linus Torvalds wrote:
> because the module locking is pure and utter crap. It uses one hug lock 
> that it tries to hold for a long time, rather than protecting just the 
> parts it needs.

Originally it was a big lock around module loading.  That was simple, and
simple is good.

Sure, that simplicity has been eroded, but "crap" is harsh.
 
> Rusty's fix is to just drop the lock around use_module(), and it seems to 
> work. It's may be right for 'use_module()', but totally wrong from a 
> conceptual locking standpoint, though - dropping the lock in the middle of 
> module loading may well "work", but who the hell knows what it really 
> results in?

I do.  If I didn't think so, I wouldn't have pushed the patch.

> IOW, it's one of those "this works, but it's very wrong" things. It makes 
> the whole module_mutex pretty much a random thing with even less semantics 
> than it has now. Right now it has some clear area that it protects - the 
> area may be too _big_, but at least it makes some amount of sense.

See, this I agree with, but you could have said this in far fewer words and
much more politely.

As posted, I had a patch to clean up the locking.  Seems you ignored it.

> It's entirely possible that an interim fix (if we can't just fix the 
> locking) is to _not_ use "strong_try_module_get()" at all, but instead 
> just use "try_module_get()", and then after we've dropped the 
> module_mutex, but _before_ we call the "init" function for the module, we 
> wait for all the modules that this module depends on.

No, those modules could still fail init.

> Doesn't that sound like the logical thing to do? And it wouldn't change 
> any locking.

No, it sounds wrong, complex and fundamentally broken.

Rusty.
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