linux-kernel - Re: [patch 2/2] Sort module list by pointer address to get coherent sleepable seq

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Date:	Tue, 21 Aug 2007 10:08:29 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/2] Sort module list by pointer address to get
	coherent sleepable seq_file iterators

On Mon, 2007-08-20 at 16:26 -0400, Mathieu Desnoyers wrote:
> plain text document attachment (module.c-sort-module-list.patch)
> A race that appears both in /proc/modules and in kallsyms: if, between the
> seq file reads, the process is put to sleep and at this moment a module is
> or removed from the module list, the listing will skip an amount of
> modules/symbols corresponding to the amount of elements present in the unloaded
> module, but at the current position in the list if the iteration is located
> after the removed module.
> 
> The cleanest way I found to deal with this problem is to sort the module list.
> We can then keep the old struct module * as the old iterator, knowing the it may
> be removed between the seq file reads, but we only use it as "get next". If it
> is not present in the module list, the next pointer will be used.
> 
> By doing this, removing a given module will now only fuzz the output related to
> this specific module, not any random module anymore. Since modprobe uses
> /proc/modules, it might be important to make sure multiple concurrent running
> modprobes won't interfere with each other.

You've reduced, but not eliminated, the problem.  A new module inserted
is quite likely to reuse the same address.

I don't have a real problem with this patch, but I'm wondering if the
problem is theoretical or demonstrated.

Rusty.


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