Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752249AbdHROT6 (ORCPT <rfc822;w@1wt.eu>);
        Fri, 18 Aug 2017 10:19:58 -0400
Received: from mx1.redhat.com ([209.132.183.28]:58216 "EHLO mx1.redhat.com"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1750709AbdHROTz (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 18 Aug 2017 10:19:55 -0400
DMARC-Filter: OpenDMARC Filter v1.3.2 mx1.redhat.com B56D881DF3
Authentication-Results: ext-mx01.extmail.prod.ext.phx2.redhat.com; dmarc=none (p=none dis=none) header.from=redhat.com
Authentication-Results: ext-mx01.extmail.prod.ext.phx2.redhat.com; spf=fail smtp.mailfrom=joe.lawrence@redhat.com
Subject: Re: [PATCH v4] livepatch: introduce shadow variable API
To: Petr Mladek <pmladek@suse.com>, Nicolai Stange <nstange@suse.de>
Cc: 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>
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>
From: Joe Lawrence <joe.lawrence@redhat.com>
Organization: Red Hat
Message-ID: <5f769d8c-f539-ebdc-359b-ef42dc7c019b@redhat.com>
Date: Fri, 18 Aug 2017 10:19:54 -0400
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101
 Thunderbird/52.2.0
MIME-Version: 1.0
In-Reply-To: <20170818140445.GC25223@pathway.suse.cz>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.25]); Fri, 18 Aug 2017 14:19:55 +0000 (UTC)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 1881
Lines: 53

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().

> 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.

-- Joe