Messages in this thread | | | From | "Chen, Dennis (SRDC SW)" <> | Subject | RE: [PATCH 1/2] Refine mutex and rcu method in module.c, kernel<3.2.9> | Date | Thu, 8 Mar 2012 12:18:19 +0000 |
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From: Rusty Russell [mailto:rusty@ozlabs.org] Sent: Thursday, March 08, 2012 5:18 PM To: Chen, Dennis (SRDC SW); linux-kernel@vger.kernel.org Cc: Chen, Dennis (SRDC SW) 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@amd.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...
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Hi Rusty,
I known RCU doesn't protect concurrent writers, so all the writers for the global modules list has been protected by the original module_mutex in the 2-patch, I just make the scope of module_mutex become smaller as it can, for example, dynamic_debug_setup doesn't touch modules, so it should not be in the protection of module_mutex.
I am a person like to see a perfect world, especially in the kernel space:) Maybe I can write a test case to trigger something you don't expect to see while the original codes will...let's think about it
BRs, Dennis
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