Date: Fri, 22 Sep 2006 14:44:23 -0400 From: John Baldwin <jhb@freebsd.org> To: "Intron is my alias on the Internet" <mag@intron.ac> Cc: freebsd-hackers@freebsd.org Subject: Re: A Bug in linker_reference_module() ? Message-ID: <200609221444.23702.jhb@freebsd.org> In-Reply-To: <courier.45134323.00004A36@intron.ac> References: <courier.4512D623.00003440@intron.ac> <200609212136.18850.jhb@freebsd.org> <courier.45134323.00004A36@intron.ac>
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On Thursday 21 September 2006 21:57, Intron is my alias on the Internet wrote: > John Baldwin wrote: > > > On Thursday 21 September 2006 14:12, Intron is my alias on the Internet wrote: > >> Please have a look at the function linker_reference_module() in > >> /sys/kern/kern_linker.c of 7.0-CURRENT. If the module is loaded on demand, > >> why not increase its reference counter after loading? In my opinion, > >> linker_reference_module() behaves differently from linker_load_file(). > > > > This is because a new kld loaded via linker_load_module() starts off > > with a refcount of 1. Thus, if you do: > > > > linker_reference_module(...); > > ... > > linker_release_module(...); > > > > Then with the current code the release_module() call drops the reference > > count to 0 and the module is unloaded. This is the desired operation for > > reference_module/release_module. This model is commonly used in the kernel. > > For example, when creating a credential, one just does 'crget()' and later > > a 'crfree()' to free it instead of doing 'crget(); crhold()' to create one. > > This model is a little confusing. If a module is loaded on demand as > dependency, its reference counter is set to 1. And if the module is loaded > by kldload(2), its reference counter is also set to 1, though in fact > no other loaded module depends on it. kldload(2) is a way for to specify a user reference on the module, and kldunload(2) is how you drop that user reference. > Although this "shift" model can work correctly, I want to know whether > there's a more reasonable way, such as setting up an auto-unloadable flag. There is this effectively done with the userref flag. When you kldload a module, it sets userref to 1, and you can only kldunload a module when userref == 1. This lets the following work: thread 1 thread 2 kldload(foo) ... ... linker_reference_module(foo) (now foo has userref == 1 and refs == 2) kldunload(foo) ... (now foo has userref == 0 and refs == 1, kldunload reports success, but module isn't unloaded due to in-kernel reference) ... linker_release_module(foo) (now refs drops to 0 and module is actually unloaded) The simpler cases also work fine if you try them out. -- John Baldwin
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