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

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Message-ID: <7d3d23c3-1839-3e6a-27bf-85bad384e5e4@gmail.com>
Date:   Sun, 3 Apr 2022 16:02:16 +0300
From:   Pavel Skripkin <paskripkin@...il.com>
To:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Michael Straube <straube.linux@...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

Hi Fabio,

On 4/3/22 15:55, Fabio M. De Francesco wrote:
> On domenica 3 aprile 2022 14:45:49 CEST Pavel Skripkin wrote:
>> Hi Fabio,
>> 
>> On 4/3/22 15:37, Fabio M. De Francesco wrote:
>> >> > 
>> >> > drivers/staging/r8188eu/core/rtw_pwrctrl.c:379
>> >> > 
>> >> >    	if (pwrpriv->ps_processing) {
>> >> >    		while (pwrpriv->ps_processing && rtw_get_passing_time_ms(start) <= 3000)
>> >> >    			msleep(10);
>> >> >    	}
>> >> > 
>> >> 
>> >> Hm, just wondering, shouldn't we annotate load from 
>> >> pwrpriv->ps_processing with READ_ONCE() inside while loop?
>> >> IIUC compiler might want to cache first load into register and we will 
>> >> stuck here forever.
>> > 
>> > You're right. This can be cached. In situations like these one should use
>> > barriers or other API that use barriers implicitly (completions, for example).
>> > 
>> 
>> Not sure about completions, since they may sleep.
> 
> No completions in this special context. They for _sure_ might sleep. I was
> talking about general cases when you are in a loop and wait for status change.
> 
>> 
>> Also, don't think that barriers are needed here, since this code just 
>> waiting for observing value 1. Might be barrier will slightly speed up 
>> waiting thread, but will also slow down other thread
> 
> Here, I cannot help with a 100% good answer. Maybe Greg wants to say something
> about it?
> 

IMO, the best answer is just remove this loop, since it does nothing. Or 
redesign it to be more sane

It waits for ps_processing to become 0 for 3000 ms, but if 3000 ms 
expires... execution goes forward like as ps_processing was 0 from the 
beginning

Maybe it's something hw related, like wait for 3000 ms and all will be 
ok. Can't say...




With regards,
Pavel Skripkin

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