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Subject: Re: [PATCH 01/12] kernfs: add function to find kernfs_node without
 increasing ref counter
From:   Paolo Valente <paolo.valente@linaro.org>
In-Reply-To: <20181112122840.GA1429@kroah.com>
Date:   Tue, 13 Nov 2018 18:53:59 +0100
Cc:     Jens Axboe <axboe@kernel.dk>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Angelo Ruocco <angeloruocco90@gmail.com>,
        Dennis Zhou <dennis@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Liu Bo <bo.liu@linux.alibaba.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        linux-block <linux-block@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Mark Brown <broonie@kernel.org>, bfq-iosched@googlegroups.com,
        oleksandr@natalenko.name, cgroups@vger.kernel.org,
        linux-doc@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>
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Message-Id: <07A5F685-9D7D-42A7-9A10-73C658F47435@linaro.org>
References: <20181112095632.69114-1-paolo.valente@linaro.org>
 <20181112095632.69114-2-paolo.valente@linaro.org>
 <20181112122840.GA1429@kroah.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> Il giorno 12 nov 2018, alle ore 13:28, Greg Kroah-Hartman =
<gregkh@linuxfoundation.org> ha scritto:
>=20
> On Mon, Nov 12, 2018 at 10:56:21AM +0100, Paolo Valente wrote:
>> From: Angelo Ruocco <angeloruocco90@gmail.com>
>>=20
>> 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.
>>=20
>> This commit adds a function to just look for a node, without getting
>> a reference on it.
>=20
> 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.
>=20

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?
>=20
> greg k-h