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

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Date:   Mon, 12 Nov 2018 17:56:27 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Paolo Valente <paolo.valente@...aro.org>,
        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@...r.kernel.org, linux-kernel@...r.kernel.org,
        ulf.hansson@...aro.org, linus.walleij@...aro.org,
        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

On Mon, Nov 12, 2018 at 04:28:40AM -0800, Greg Kroah-Hartman wrote:
> 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.
> 
> 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?

I agree with Greg, this function is dangerous.  What's wrong with using
find_get and then doing a put once you successfully take it over, or
fail to take it over?