linux-kernel - Re: [PATCH v2 ] devcoredump : Serialize devcd

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Date:   Mon, 25 Apr 2022 21:37:41 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Johannes Berg <johannes@...solutions.net>,
        Mukesh Ojha <quic_mojha@...cinc.com>,
        linux-kernel@...r.kernel.org
Cc:     sboyd@...nel.org, rafael@...nel.org, gregkh@...uxfoundation.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v2 ] devcoredump : Serialize devcd_del work

Cc+: Kees

On Mon, Apr 25 2022 at 19:19, Johannes Berg wrote:
> On Mon, 2022-04-25 at 19:00 +0200, Thomas Gleixner wrote:
>> 
>> Johannes, can you please explain whether this immediate flush in
>> disabled_store() is really required and if so, why?
>> 
> I don't really know, as I remember that requirement (or maybe even code,
> not sure) came from Kees, who needed the lockdown.
>
> Given the use case (ChromeOS?) I'm not sure I see a need to flush all of
> them, since I guess a typical system would set the lockdown early in
> boot and hopefully not have a crash-dump around already.
>
> That said, I don't think the diagram you made works - fn() during the
> iteration is guaranteed to be invoked with a reference of its own, so
> the put_device() there can't be the last reference, only as fn() returns
> you'd put the last reference *there*, freeing it.

Bah, you are right, it's magically protected by the klist ref, which
prevents devcd from going away. Damned obvious.

This really needs comments why this all can magically "work".

Thanks,

        tglx