linux-kernel - Re: [PATCH 01/12] kernfs: add function to find kernfs

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Date:   Tue, 13 Nov 2018 18:53:59 +0100
From:   Paolo Valente <paolo.valente@...aro.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Angelo Ruocco <angeloruocco90@...il.com>,
        Dennis Zhou <dennis@...nel.org>,
        Josef Bacik <josef@...icpanda.com>,
        Liu Bo <bo.liu@...ux.alibaba.com>,
        Bart Van Assche <bvanassche@....org>,
        Johannes Weiner <hannes@...xchg.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Mark Brown <broonie@...nel.org>, bfq-iosched@...glegroups.com,
        oleksandr@...alenko.name, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, Jonathan Corbet <corbet@....net>
Subject: Re: [PATCH 01/12] kernfs: add function to find kernfs_node without
 increasing ref counter



> Il giorno 12 nov 2018, alle ore 13:28, Greg Kroah-Hartman <gregkh@...uxfoundation.org> ha scritto:
> 
> On Mon, Nov 12, 2018 at 10:56:21AM +0100, Paolo Valente wrote:
>> From: Angelo Ruocco <angeloruocco90@...il.com>
>> 
>> The kernfs pseudo file system doesn't export any function to only find
>> a node by name, without also getting a reference on it.
>> But in some cases it is useful to just locate a kernfs node, while
>> using it or not depends on some other condition.
>> 
>> This commit adds a function to just look for a node, without getting
>> a reference on it.
> 
> Eeek, that sounds really bad.  So you save off a pointer to something,
> and have no idea if that pointer now really is valid or not?  It can
> instantly disappear right afterwards.
> 

Hi Greg,
that function is invoked only in functions executed with cgroup_mutex
held.  This guarantees that nothing disappears or becomes
inconsistent.  That's why we decided to go for this optimization,
instead of doing useless gets&puts pairs.  Still, I'm not expert
enough to state whether it is impossible that, once we have defined
that function, it may then get used in some unsafe way.

So, I seem to see two options:
1) Add a comment on the function, saying that cgroup_mutex must be
   held while invoking it (I guess you won't like this one).
2) Do not define such a new function, and, in the other patches, use
   the already-available find_and_get.

Looking forward to your feedback (or of other knowledgeable people on
this issue) before proceeding to a rebased V2,
Paolo


> This feels wrong, what is the problem of having a properly reference
> counted object passed back to you that you have to create a dangerous
> function like this?
> 
> greg k-h