linux-kernel - Re: [PATCH 8/9] module: Switch to kvfree

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Date:   Tue, 30 Nov 2021 11:39:09 +0100 (CET)
From:   Miroslav Benes <mbenes@...e.cz>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
cc:     LKML <linux-kernel@...r.kernel.org>, RCU <rcu@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Daniel Axtens <dja@...ens.net>,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Theodore Y . Ts'o" <tytso@....edu>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...ymobile.com>,
        Luis Chamberlain <mcgrof@...nel.org>, jeyu@...nel.org
Subject: Re: [PATCH 8/9] module: Switch to kvfree_rcu() API

Hi,

On Wed, 24 Nov 2021, Uladzislau Rezki (Sony) wrote:

> Instead of invoking a synchronize_rcu() to free a pointer
> after a grace period we can directly make use of new API
> that does the same but in more efficient way.
> 
> CC: Luis Chamberlain <mcgrof@...nel.org>
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@...il.com>
> ---
>  kernel/module.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 84a9141a5e15..f404f0c9f385 100644
> --- 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.

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.

Miroslav