linux-kernel - Re: [PATCH-next v2 1/2] driver core: introduce get_device_unless

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Date:   Sat, 28 Jan 2023 11:43:20 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Zhong Jinghua <zhongjinghua@...wei.com>
Cc:     jejb@...ux.ibm.com, martin.petersen@...cle.com, hare@...e.de,
        bvanassche@....org, emilne@...hat.com,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        yi.zhang@...wei.com, yukuai3@...wei.com
Subject: Re: [PATCH-next v2 1/2] driver core: introduce
 get_device_unless_zero()

On Sat, Jan 28, 2023 at 05:41:45PM +0800, Zhong Jinghua wrote:
> When the dev reference count is 0, calling get_device will go from 0 to 1,

You can NOT have a device reference count that is 0.  If you do, you are
doing something really really wrong, and there's a bug somewhere else.

> which will cause errors in some place of the kernel.

It's already an error in the kernel that tries to increment a reference
count of 0 as that device is already freed and you are working with
memory that is not present.

> So introduce a
> get_devcie_unless_zero method that returns NULL when the dev reference
> count is 0.

No, this is not ok, sorry, please never do this.  Fix the caller.

thanks,

greg k-h