linux-kernel - Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free resources tied to the file

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Date:   Wed, 28 Jun 2023 11:18:20 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Christian Brauner <brauner@...nel.org>, gregkh@...uxfoundation.org,
        peterz@...radead.org, lujialin4@...wei.com,
        lizefan.x@...edance.com, hannes@...xchg.org, mingo@...hat.com,
        ebiggers@...nel.org, oleg@...hat.com, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH 1/2] kernfs: add kernfs_ops.free operation to free
 resources tied to the file

On Wed, Jun 28, 2023 at 11:02 AM Tejun Heo <tj@...nel.org> wrote:
>
> On Wed, Jun 28, 2023 at 07:35:20PM +0200, Christian Brauner wrote:
> > > To summarize my understanding of your proposal, you suggest adding new
> > > kernfs_ops for the case you marked (1) and change ->release() to do
> > > only (2). Please correct me if I misunderstood. Greg, Tejun, WDYT?
> >
> > Yes. I can't claim to know all the intricate implementation details of
> > kernfs ofc but this seems sane to me.
>
> This is going to be massively confusing for vast majority of kernfs users.
> The contract kernfs provides is that you can tell kernfs that you want out
> and then you can do so synchronously in a finite amount of time (you still
> have to wait for in-flight operations to finish but that's under your
> control). Adding an operation which outlives that contract as something
> usual to use is guaranteed to lead to obscure future crnashes. For a
> temporary fix, it's fine as long as it's marked clearly but please don't
> make it something seemingly widely useable.
>
> We have a long history of modules causing crashes because of this. The
> severing semantics is not there just for fun.

I'm sure there are reasons things are working as they do today. Sounds
like we can't change the ->release() logic from what it is today...
Then the question is how do we fix this case needing to release a
resource which can be released only when there are no users of the
file? My original suggestion was to add a kernfs_ops operation which
would indicate there are no more users but that seems to be confusing.
Are there better ways to fix this issue?
Thanks,
Suren.

>
> Thanks.
>
> --
> tejun