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

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Message-Id: <201006021233.09140.rusty@rustcorp.com.au>
Date:	Wed, 2 Jun 2010 12:33:05 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Brandon Philips <brandon@...p.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.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 Wed, 2 Jun 2010 11:40:29 am Brandon Philips wrote:
> On 16:51 Tue 01 Jun 2010, Linus Torvalds wrote:
> > On Tue, 1 Jun 2010, Brandon Philips wrote:
> > > When I tested a Kernel with Rusty's modules branch pulled onto
> > > 2.6.35-rc1 I got duplicate sysfs path errors:
> > Hmm. Yeah, the module_mutex used to be held across the whole "find -> add" 
> > state, but isn't any more.
> 
> Right.
> 
> > > To fix this we need to make sure that we only have one copy of a
> > > module going through load_module at a time.  Patch suggestion
> > > follows which boots without errors. I am sure there is a better
> > > way to do what it does ;)
> > 
> > I think Rusty may have made the lock a bit _too_ finegrained there,
> > and didn't add it to some places that needed it. It looks, for
> > example, like PATCH 1/2 actually drops the lock in places where it's
> > needed ("find_module()" is documented to need it, but now
> > load_module() didn't hold it at all when it did the find_module()).
> 
> Right, I noticed that too and held the lock in the patch I sent.
> 
> > Rather than adding a new "module_loading" list, I think we should be
> > able to just use the existing "modules" list, and just fix up the
> > locking a bit.
> >
> > In fact, maybe we could just move the "look up existing module" a
> > bit later - optimistically assuming that the module doesn't exist,
> > and then just undoing the work if it turns out that we were wrong,
> > just before adding ourselves to the list.
> >
> > A patch something like the appended (TOTALLY UNTESTED!)
> 
> FWIW, I tried this same idea initially and it breaks because the
> kobject EEXIST is coming from mod_sysfs_init() which happens further
> up in load_module() before the list_add_rcu().

I'll take a look.  Linus was right that my lock reduction was overzealous.

The kobject error should be harmless.

I have a simplified version of your problem (see testing patch); I'll
see what I can fix...

Thanks!
Rusty.
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