linux-kernel - Re: [linux-pm] Is it supposed to be ok to call del

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Message-ID: <4BF0F3FF.2010603@crca.org.au>
Date:	Mon, 17 May 2010 17:45:03 +1000
From:	Nigel Cunningham <ncunningham@...a.org.au>
To:	Alan Stern <stern@...land.harvard.edu>
CC:	linux-kernel <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-pm <linux-pm@...ts.linux-foundation.org>,
	Matt Reimer <mattjreimer@...il.com>
Subject: Re: [linux-pm] Is it supposed to be ok to call del_gendisk while
 userspace is frozen?

Hi.

On 17/05/10 12:22, Alan Stern wrote:
> On Mon, 17 May 2010, Nigel Cunningham wrote:
>
>>>> I object to the patch.
>>>>
>>>> Tell the patch it ought to exit once thawed, by all means.
>>>
>>> I'm not sure what you mean.  Care to explain?
>>
>> I mean "Set up some sort of flag that it can look at once thawed at
>> resume time, and use that to tell it to exit at that point."
>
> Doesn't the patch do exactly that?  The "flag" is set by virtue of the
> fact that this is part of del_gendisk -- which means the disk is being
> unregistered and hence the writeback thread will exit shortly.
>
>>>> Make the patch unfreezeable to begin with, by all means.
>>>
>>> That wouldn't work.
>>
>> Why not?
>
> It would be nice to know exactly why.  Perhaps the underlying problem
> can be fixed.
>
>>>> If you know a disk is going to be unregistered during resume,
>>>
>>> How do we check that, exactly?
>>
>> Well, if you can figure out that you need to go down this path at this
>> point in the process, you must be able to apply the same logic to come
>> to the same conclusion earlier in the process.
>
> That's not true.  You don't know that a device is going to be unplugged
> until it actually _is_ unplugged.

Sorry - I got unregistered during suspend (instead of resume) in my 
head. That said, I'd argue that we should be...

1) Syncing all the data at the start of the suspend/hibernate, so 
there's nothing for the workthread to do if we do del_gendisk.
2) Telling things to exit if we do find the device is gone away at 
resume time, but not relying on the going-away happening until post 
process thaw, for a couple of reasons:
- Potential for races/confusion/mess etc in having $random process 
thawing other processes. Only the thread doing the suspend/hibernate 
should be freezing/thawing.
- We're dealing with the symptom, not the cause. Almost always a bad idea.

Regards,

Nigel
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