linux-kernel - Re: [PATCH 1/2] Refine mutex and rcu method in module.c, kernel<3.2.9>

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Message-ID: <87ipifwiwh.fsf@rustcorp.com.au>
Date:	Thu, 08 Mar 2012 19:48:06 +1030
From:	Rusty Russell <rusty@...abs.org>
To:	"Chen\, Dennis \(SRDC SW\)" <Dennis1.Chen@....com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>
Cc:	"Chen\, Dennis \(SRDC SW\)" <Dennis1.Chen@....com>
Subject: Re: [PATCH 1/2] Refine mutex and rcu method in module.c, kernel<3.2.9>

On Wed, 7 Mar 2012 14:51:06 +0000, "Chen, Dennis (SRDC SW)" <Dennis1.Chen@....com> wrote:
> 1. Narrow down the granularity of mutex_lock/ mutex_unlock
> 2. Replace some unnecessary mutex_lock/mutex_unlock pairs with RCU
> 3. Refine the consistent calling style of RCU functioan 

Hi Dennis,

        This follows a logical evolution, where we wean off the mutex,
but AFAICT this is lost in the noise.  Taking the mutex might be naive,
but adding or removing a module is the slow path.  Or am I missing
something?

> -
> -       /* Now sew it into the lists so we can get lockdep and oops
> -        * info during argument parsing.  No one should access us, since
> -        * strong_try_module_get() will fail.
> -        * lockdep/oops can run asynchronous, so use the RCU list insertion
> -        * function to insert in a way safe to concurrent readers.
> -        * The mutex protects against concurrent writers.
> -        */
> -       mutex_lock(&module_mutex);
> +        
> +        /* Concurrent writers for the global modules list are protected by RCU*/
>         if (find_module(mod->name)) {
>                 err = -EEXIST;
>                 goto unlock;
>         }

RCU does not protect concurrent writers:

> -
> +        
>         /* This has to be done once we're sure module name is unique. */
>         dynamic_debug_setup(info.debug, info.num_debug);

Now this is racy...

Cheers,
Rusty.
-- 
  How could I marry someone with more hair than me?  http://baldalex.org
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