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SubjectRe: [PATCH 8/9] module: Switch to kvfree_rcu() API
On Tue, Nov 30, 2021 at 12:04:19PM +0100, Sebastian Andrzej Siewior wrote:
> On 2021-11-30 11:39:09 [+0100], Miroslav Benes wrote:
> > > --- a/kernel/module.c
> > > +++ b/kernel/module.c
> > > @@ -4150,8 +4150,7 @@ static int load_module(struct load_info *info, const char __user *uargs,
> > > ddebug_cleanup:
> > > ftrace_release_mod(mod);
> > > dynamic_debug_remove(mod, info->debug);
> > > - synchronize_rcu();
> > > - kfree(mod->args);
> > > + kvfree_rcu(mod->args);
> > > free_arch_cleanup:
> > > cfi_cleanup(mod);
> > > module_arch_cleanup(mod);
> >
> > hm, if I am not missing something, synchronize_rcu() is not really
> > connected to kfree(mod->args) there. synchronize_rcu() was added a long
> > time ago when kernel/module.c removed stop_machine() from the code and
> > replaced it with RCU to protect (at least?) mod->list. You can find
> > list_del_rcu(&mod->list) a couple of lines below.
>
> so instead synchronize_rcu() + kfree() you could do
> call_rcu(&mod->args->rcu, kfree()) but since you have no RCU-head around
> in args you wait for the grace period and then invoke kfree.
>
> kvfree_rcu() is somehow like call_rcu() + kfree() but without the needed
> RCU-head.
> So you avoid waiting for the grace period but mod->args is freed later,
> after as expected.
>
> > And yes, one could ask how this all works. The error/cleanup sequence in
> > load_module() is a giant mess... well, load_module() is a mess too, but
> > the error path is really not nice.
>
> Well, spring is coming :)
>
Indeed that error path sequence is terrible. I will double check if that
synchronize_rcu() + kfree() is related to any RCU protection and freeing.

If it is not i will drop this patch.


Thanks!

--
Vlad Rezki

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