Messages in this thread | | | Date | Thu, 12 Feb 2009 20:46:40 -0500 (EST) | From | Tim Abbott <> | Subject | Re: [PATCH 6/7] Ksplice: Export symbols needed for Ksplice |
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On Wed, 4 Feb 2009, Rusty Russell wrote:
> On Saturday 06 December 2008 10:33:59 Jeff Arnold wrote: > > From: Tim Abbott <tabbott@mit.edu> > > > > Ksplice uses several functions from module.c in order to resolve > > symbols and implement dependency handling. Calling these functions > > requires holding module_mutex, so it is exported. > > I have fought every attempt to export that. It was long a cause of misery > and pain; people saw the lock and so started grabbing it. > > Any chance we can find another way?
It is definitely possible, but it wouldn't be pretty. There are a few things that we use module_mutex for:
(1) Preventing races involving modules being unloaded during run-pre matching. Functions such as kallsyms_on_each_symbol, find_symbol, __module_text_address, __module_address, and find_module that are used to obtain a module or a symbol can only be used within preempt_disable or while holding module_mutex. Further, the values that they return can only be used in the same such context, unless one module_gets the relevant module in that context.
So, we'd change/add wrappers to a bunch of these functions to do their own preempt_disable or grabbing of module_mutex, and to module_get the relevant module before they return. We'd also need to change the Ksplice code to module_put whenever we stop referencing a symbol. Since a lot of the Ksplice kernel code is related to resolving symbols, I suspect this latter part would be somewhat messy.
(2) Calling use_module. use_module needs to be called while holding module_mutex because it changes the modules_which_use_me lists.
We could change this by creating a modified version of use_module that grabs module_mutex itself; perhaps renaming the current use_module to __use_module.
(3) Preventing races involving two different Ksplice updates being applied at the same time.
This one is easy; we could just create a new ksplice_mutex.
-Tim Abbott
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