Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1751671AbdHROqq convert rfc822-to-8bit (ORCPT <rfc822;w@1wt.eu>);
        Fri, 18 Aug 2017 10:46:46 -0400
Received: from mx2.suse.de ([195.135.220.15]:41907 "EHLO mx1.suse.de"
        rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP
        id S1751489AbdHROqp (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 18 Aug 2017 10:46:45 -0400
From: Nicolai Stange <nstange@suse.de>
To: Joe Lawrence <joe.lawrence@redhat.com>
Cc: Petr Mladek <pmladek@suse.com>, Nicolai Stange <nstange@suse.de>,
        live-patching@vger.kernel.org, linux-kernel@vger.kernel.org,
        Josh Poimboeuf <jpoimboe@redhat.com>, Jessica Yu <jeyu@redhat.com>,
        Jiri Kosina <jikos@kernel.org>, Miroslav Benes <mbenes@suse.cz>
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API
References: <1502740963-31310-1-git-send-email-joe.lawrence@redhat.com>
        <1502740963-31310-2-git-send-email-joe.lawrence@redhat.com>
        <87pobtj8hu.fsf@suse.de> <20170818140445.GC25223@pathway.suse.cz>
        <5f769d8c-f539-ebdc-359b-ef42dc7c019b@redhat.com>
Date: Fri, 18 Aug 2017 16:46:41 +0200
In-Reply-To: <5f769d8c-f539-ebdc-359b-ef42dc7c019b@redhat.com> (Joe Lawrence's
        message of "Fri, 18 Aug 2017 10:19:54 -0400")
Message-ID: <87378pgcha.fsf@suse.de>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8BIT
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2099
Lines: 59

Joe Lawrence <joe.lawrence@redhat.com> writes:

> On 08/18/2017 10:04 AM, Petr Mladek wrote:
>> On Fri 2017-08-18 15:44:29, Nicolai Stange wrote:
>>> Joe Lawrence <joe.lawrence@redhat.com> writes:
>>>
>>> <snip>
>>>> +
>>>> +/**
>>>> + * klp_shadow_get() - retrieve a shadow variable data pointer
>>>> + * @obj:	pointer to parent object
>>>> + * @id:		data identifier
>>>> + *
>>>> + * Return: the shadow variable data element, NULL on failure.
>>>> + */
>>>> +void *klp_shadow_get(void *obj, unsigned long id)
>>>> +{
>>>> +	struct klp_shadow *shadow;
>>>> +
>>>> +	rcu_read_lock();
>>>> +
>>>> +	hash_for_each_possible_rcu(klp_shadow_hash, shadow, node,
>>>> +				   (unsigned long)obj) {
>>>> +
>>>> +		if (klp_shadow_match(shadow, obj, id)) {
>>>> +			rcu_read_unlock();
>>>> +			return shadow->data;
>>>
>>> I had to think a moment about what protects shadow from getting freed by
>>> a concurrent detach after that rcu_read_unlock(). Then I noticed that if
>>> obj and the livepatch are alive, then so is shadow, because there
>>> obviously hasn't been any reason to detach it.
>>>
>>> So maybe it would be nice to have an additional comment at
>>> klp_shadow_detach() that it's the API user's responsibility not to use a
>>> shadow instance after detaching it...
>
> Nicolai, I can add something like "This function releases the memory for
> this shadow variable instance, callers should stop referencing it
> accordingly."  Similar text for klp_shadow_detach_all().

Perfect, thanks!


>> Good point. In fact, it might make sense to rename the functions:
>> 
>>      attach -> create
>>      detach -> destroy
>> 
>> The name detach suggests that the variable is just not connected to
>> the parent object but that it is still accessible/usable.
>
> FWIW, kpatch calls them "kpatch_shadow_alloc" and "kpatch_shadow_free".
> Now that it's clear that we're not going separate shadow variable
> allocation from hash table insertion, going back to alloc/create and
> destroy/free is fine w/me.

-- 
SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)