netdev - Re: [PATCH] nfc/nci: fix task hung in nfc_targets

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Date: Mon, 15 Jan 2024 20:08:58 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>,
        Edward Adam Davis <eadavis@...com>,
        syzbot+2b131f51bb4af224ab40@...kaller.appspotmail.com
Cc: davem@...emloft.net, edumazet@...gle.com, gregkh@...uxfoundation.org,
        hdanton@...a.com, kuba@...nel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, pabeni@...hat.com, stern@...land.harvard.edu,
        syzkaller-bugs@...glegroups.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH] nfc/nci: fix task hung in nfc_targets_found

On 2024/01/15 18:36, Krzysztof Kozlowski wrote:
>> diff --git a/net/nfc/nci/core.c b/net/nfc/nci/core.c
>> index 6c9592d05120..9a277228a875 100644
>> --- a/net/nfc/nci/core.c
>> +++ b/net/nfc/nci/core.c
>> @@ -145,6 +145,8 @@ inline int nci_request(struct nci_dev *ndev,
>>  {
>>  	int rc;
>>  
>> +	if (test_bit(NCI_UNREG, &ndev->flags))
>> +		return -ENODEV;
> 
> nci_close_device() clears the NCI_UP, which is tested here, just after
> acquiring mutex. And there is explicit comment about it just below your
> code. Why it is not relevant?

Because the deadlock happens at mutex_lock(&ndev->req_lock), which is
before test_bit(NCI_UP, &ndev->flags) is called. Please see
https://lkml.kernel.org/r/d314e471-0251-461e-988d-70add0c6ebf6@I-love.SAKURA.ne.jp .

> 
> Your code looks really unnecessary, at least with that code flow from
> commit msg. Especially considering you do it outside of mutex, so how
> does it solve anything?
> 
> Best regards,
> Krzysztof
>