linux-kernel - Re: staging: r8188eu: how to handle nested mutex under spinlock

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Date:   Sun, 3 Apr 2022 13:08:35 +0200
From:   Michael Straube <straube.linux@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Larry Finger <Larry.Finger@...inger.net>,
        Phillip Potter <phil@...lpotter.co.uk>,
        "open list:STAGING SUBSYSTEM" <linux-staging@...ts.linux.dev>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: staging: r8188eu: how to handle nested mutex under spinlock

On 4/3/22 12:49, Fabio M. De Francesco wrote:
> On domenica 3 aprile 2022 12:43:04 CEST Fabio M. De Francesco wrote:
>> On sabato 2 aprile 2022 22:47:27 CEST Michael Straube wrote:
>>> Hi all,
>>>
>>> smatch reported a sleeping in atomic context.
>>>
>>> rtw_set_802_11_disassociate() <- disables preempt
>>> -> _rtw_pwr_wakeup()
>>>      -> ips_leave()
>>>
>>> rtw_set_802_11_disassociate() takes a spinlock and ips_leave() uses a
>>> mutex.
>>>
>>> I'm fairly new to the locking stuff, but as far as I know this is not a
>>> false positive since mutex can sleep, but that's not allowed under a
>>> spinlock.
>>>
>>> What is the best way to handle this?
>>> I'm not sure if converting the mutex to a spinlock (including all the
>>> other places where the mutex is used) is the right thing to do?
>>>
>>> thanks,
>>> Michael
>>>
>> Hi Michael,
>>
>> No, this is a false positive: ips_leave is never called under spinlocks.
>> Some time ago I blindly trusted Smatch and submitted a patch for what you
>> are reporting just now again. Soon after submission I realized it and
>> then I had to ask Greg to discard my patch.
>>
>> Please read the related thread:
>>
>> [PATCH] staging: r8188eu: Use kzalloc() with GFP_ATOMIC in atomic context
>> https://lore.kernel.org/lkml/20220206225943.7848-1-fmdefrancesco@gmail.com/
>>
>> Thanks,
>>
>> Fabio
> 
> I'm sorry, the correct link is the following:
> [PATCH v2 2/2] staging: r8188eu: Use kzalloc() with GFP_ATOMIC in atomic context
> https://lore.kernel.org/lkml/20220208180426.27455-3-fmdefrancesco@gmail.com/
> 
> Fabio
> 

Hi Fabio,

Ah I see now, thanks. Well, I think the code is not very clear and easy 
to follow here. Perhaps we should refactor this area someday to avoid 
future confusions.

regards,
Michael