linux-kernel - Re: [PATCH 1/2] devcoredump: Remove devcoredump device if failing device is gone

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Date: Wed, 31 Jan 2024 22:52:59 +0530
From: Mukesh Ojha <quic_mojha@...cinc.com>
To: Johannes Berg <johannes@...solutions.net>,
        Rodrigo Vivi
	<rodrigo.vivi@...el.com>, <linux-kernel@...r.kernel.org>
CC: Jose Souza <jose.souza@...el.com>,
        Maarten Lankhorst
	<maarten.lankhorst@...ux.intel.com>,
        Greg Kroah-Hartman
	<gregkh@...uxfoundation.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>
Subject: Re: [PATCH 1/2] devcoredump: Remove devcoredump device if failing
 device is gone



On 1/29/2024 11:18 PM, Johannes Berg wrote:
> On Fri, 2024-01-26 at 10:11 -0500, Rodrigo Vivi wrote:
>> Make dev_coredumpm a real device managed helper, that not only
>> frees the device after a scheduled delay (DEVCD_TIMEOUT), but
>> also when the failing/crashed device is gone.
>>
>> The module remove for the drivers using devcoredump are currently
>> broken if attempted between the crash and the DEVCD_TIMEOUT, since
>> the symbolic sysfs link won't be deleted.
> 
> Hmm, is it a problem to remove a whole dev when it still has some link
> here? Maybe we could just make the link be managed/auto-removed?
> 
> Probably regardless of that you should change the comment in
> devcd_dev_release() since it's no longer a concern?
> 
>> On top of that, for PCI devices, the unbind of the device will
>> call the pci .remove void function, that cannot fail. At that
>> time, our device is pretty much gone, but the read and free
>> functions are alive trough the devcoredump device and they
>                        ^ through, I guess
> 
>> can get some NULL dereferences or use after free.
> 
> Not sure I understand this part, how's this related to PCI's .remove?
> 
>> So, if the failing-device is gone let's also request for the
>> devcoredump-device removal using the same mod_delayed_work
>> as when writing anything through data. The flush cannot be
>> used since it is synchronous and the devcd would be surely
>> gone right before the mutex_unlock on the next line.
> 
> Can we just decouple it instead and remove the symlink? Which is kind of
> what the comment in devcd_dev_release() says but at the time I wasn't
> aware of all the devm mechanics etc.

Are we going to do this ?

-Mukesh

> 
> I'm thinking this might be annoying in certain recovery cases, e.g.
> iwlwifi uses this but may sometimes unbind/rebind itself to recover from
> certain errors, and that'd make the FW dumps disappear.
> 
> johannes
>