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Date: Tue, 16 Aug 2022 14:51:50 -0600
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From: Jacob Moody <moody@mail.posixcafe.org>
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X-Bullshit: agile flexible deep-learning dependency-oriented full-stack shader 
Subject: Re: [9front] fix race condition on writes to #v/vgactl
Reply-To: 9front@9front.org
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On 8/16/22 14:44, qwx@sciops.net wrote:
> On Tue Aug 16 08:12:41 +0200 2022, qwx@sciops.net wrote:
>> Hello,
>>
>> I've been experiencing a seemingly impossible kernel panic when
>> messing with vgactl.  Occasionally, writing multiple commands to
>> /dev/vgactl rapidly may cause an assert to fail
>> (/sys/src/9/port/devdraw.c:/assert).  The panic is perfectly
>> reproducible by sending `softscreen off' commands quickly (tested on
>> x61t):
>>
>> 	; for(i in `{seq 10}) echo softscreen off >/dev/vgactl
>>
>> The panic occurs within the first few iterations.  Moving some locks
>> out of individual functions to hold them for the entire step fixes
>> this.  The patch below works, but qunlock may be called without the
>> drawlock being held, I need to recheck and clean up before commiting
>> anything.  Credit to moody for suggesting looking in this direction!
> 
> Here's the final patch, mostly the same.  I'll commit this later
> unless there are any objections.
> 
> Thanks,
> qwx
> 

Checks out to me.