linux-kernel - Re: [PATCH net-next 1/2] nfc: s3fwrn5: fix order of freeing resources

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Date:   Thu, 29 Sep 2022 00:51:02 -0700
From:   Dmitry Torokhov <dmitry.torokhov@...il.com>
To:     Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Krzysztof Opasiak <k.opasiak@...sung.com>
CC:     netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/2] nfc: s3fwrn5: fix order of freeing resources

On September 29, 2022 12:42:08 AM PDT, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
>On 29/09/2022 09:37, Dmitry Torokhov wrote:
>> On September 29, 2022 12:27:19 AM PDT, Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
>>> On 29/09/2022 09:26, Krzysztof Kozlowski wrote:
>>>> On 29/09/2022 07:04, Dmitry Torokhov wrote:
>>>>> Caution needs to be exercised when mixing together regular and managed
>>>>> resources. In case of this driver devm_request_threaded_irq() should
>>>>> not be used, because it will result in the interrupt being freed too
>>>>> late, and there being a chance that it fires up at an inopportune
>>>>> moment and reference already freed data structures.
>>>>
>>>> Non-devm was so far recommended only for IRQF_SHARED, not for regular
>>>> ones.
>> 
>> If we are absolutely sure there is no possibility of interrupts firing then devm
>> should be ok, but it is much safer not to use it. Or use custom devm actions
>> to free non-managed resources.
>
>I am not sure and the pattern itself is a bit risky, I agree. However
>the driver calls s3fwrn5_remove() which then calls
>s3fwrn5_phy_power_ctrl() which cuts the power via GPIO pin. I assume
>that the hardware should stop generating interrupts at this point.

Ok, fair enough. The GPIO is mandatory, so I guess the chip will be disabled.

>
>> 
>>>> Otherwise you have to fix half of Linux kernel drivers... 
>> 
>> Yes, if they are doing the wrong thing.
>
>What I meant, that this pattern appears pretty often. If we agree that
>this driver has a risky pattern (hardware might not be off after
>remove() callback), then we should maybe document it somewhere and
>include it in usual reviews.

I have been saying that mixing managed and non managed resource is
dangerous for years. Disabling clocks before shutting off interrupts, freeing
memory, etc, is not safe. The best approach with devm is all or nothing.

Thanks.

-- 
Dmitry