linux-kernel - Re: [PATCH] Fix race in efi variable delete code

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Date:	Thu, 25 Jan 2007 22:00:29 -0800
From:	Andrew Morton <akpm@...l.org>
To:	Matt Domsch <Matt_Domsch@...l.com>
Cc:	Prarit Bhargava <prarit@...hat.com>, linux-kernel@...r.kernel.org,
	matthew.e.tolentino@...el.com, anil.s.keshavamurthy@...el.com
Subject: Re: [PATCH] Fix race in efi variable delete code

On Thu, 25 Jan 2007 16:20:56 -0600
Matt Domsch <Matt_Domsch@...l.com> wrote:

> Fix race when deleting an EFI variable and issuing another EFI command on the
> same variable.  The removal of the variable from the efivars_list should be
> done in efivar_delete and not delayed until the kobject release.
> 
> Furthermore, remove the item from the list at module unload time, and
> use list_for_each_entry_safe() rather than list_for_each_safe() for readability.
> 

Does it actually need to use the _safe variant?  That's only needed if the
body of the loop can do list_del() and afaict that doesn't happen here.

>  static void __exit
>  efivars_exit(void)
>  {
> -	struct list_head *pos, *n;
> +	struct efivar_entry *entry, *n;
>  
> -	list_for_each_safe(pos, n, &efivar_list)
> -		efivar_unregister(get_efivar_entry(pos));
> +	list_for_each_entry_safe(entry, n, &efivar_list, list) {
> +		spin_lock(&efivars_lock);
> +		list_del(&entry->list);
> +		spin_unlock(&efivars_lock);
> +		efivar_unregister(entry);
> +	}

That's not exactly a thing of beauty, sorry ;)

Given that the code is single-threaded here, there's nothing to race
against and I don't think we strictly need any locking at all.  But
consistency is OK.  Given the locking here I'm not sure that the code would
be safe against concurrent removes anyway.

A more idiomatic implementation would do:

	while (!list_empty(&efivar_list)) {
		struct efivar_entry *entry = list_entry(...);
		list_del(...)
	}

Anyway.  Stuff to think about on a rainy day...
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