linux-kernel - Re: [PATCH] mtd: use refcount to prevent corruption

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Date:   Thu, 28 Jan 2021 21:23:29 +0100 (CET)
From:   Richard Weinberger <richard@....at>
To:     Tomas Winkler <tomas.winkler@...el.com>
Cc:     Miquel Raynal <miquel.raynal@...tlin.com>,
        Vignesh Raghavendra <vigneshr@...com>,
        linux-mtd <linux-mtd@...ts.infradead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mtd: use refcount to prevent corruption

Tomas,

----- Ursprüngliche Mail -----
>> As Richard was saying, we are really open to enhance MTD refcounting.
>> 
>> However, the issue you are facing is, IMHO, not related to MTD but to MFD.
>> There should be a way to avoid MFD to vanish by taking a reference of it
>> through mtd->_get_device(). I don't think addressing the case where MFD
>> vanishes while MTD (as a user) is still active is the right approach.
> 
> I think it won't work because MFD sub-driver remove() is called   and it must
> succeed because the main device  is not accessible unlike glueubi which just
> returns -EBUSY.

Well, the trick in glubi (and other MTDs with "hotplug" support) is not to reject
removal of the sub-device. ->_put_device() is of return type void.
The key is grabbing a reference on the sub-device in ->_get_device() such that
the layer below doesn't even try to remove while the MTD is in use.

> so we postpone the mtd unregister to  mtd_info->_put_device()  but it that state
> we have nothing to hold
> on as the device is gone in remove()
> User will fail anyway, as the underlying device is not functional in that state.
> Anyway I've tried your suggestion, the kernel is crashing, hope I haven't done
> some silly bug.

Can you point us to the affected code?
This would help a lot to understand the issue better.
I'm sure we can find a solution.

Thanks,
//richard