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Date: Tue, 8 Jun 2010 12:19:54 +0200
From: Florian Mickler <florian@mickler.org>
To: Arve =?ISO-8859-15?Q?Hj=F8nnev=E5g?= <arve@android.com>
Cc: Alan Stern <stern@rowland.harvard.edu>, tytso@mit.edu,
       Alan Cox <alan@lxorguk.ukuu.org.uk>,
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       Ingo Molnar <mingo@elte.hu>, Felipe Balbi <felipe.balbi@nokia.com>,
       Arjan van de Ven <arjan@infradead.org>
Subject: Re: [linux-pm] suspend blockers & Android integration
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On Mon, 7 Jun 2010 20:05:56 -0700
Arve Hj?nnev?g <arve@android.com> wrote:
Hi,

> 
> If you read an event that occurred after you blocked the task
> freezing, then tasks will never get frozen again (until more events
> occur). I think my original description was less confusing, but it
> seems you got completely distracted by my use of block and unblock
> suspend when referring to the user space api.

Here is how I understood Alan's approach: 

Userspace manager (UM) does:

<...continuation of function A>
5) unblock from reading a wakeup from wakeupevents-fd
6) thaw userspace
7) return
</function A>

[userspace sees there is an event; blocks suspend at UM; processes
event; consume wakeupevent at UM; unblock suspend at UM;] 

Unblocking the last suspend-blocker at the UM starts function A:

<function A>
1) non-blocking read of wakeup-events-fd (refills wakeupevents)
2) if all wakeupevents are consumed: 
	3a) freeze userspace	
   else
	3b) /* wait for userspace to unblock
	suspend again... this should take care of the races? */ return;
4) blocking read of wakeupevents-fd
<...for continuation see above>

You mitigate the race by freezing and unfreezing userspace. If there
occur wakeups between 3a) and 4) you will have frozen userspace in
vain. 

So I think the feasibility of this solution depends on the performance
of freezing/thawing userspace. I can't judge that.

Also I _think_ this is racefree as long as you have the UM properly
serialized. Or did I overlook something?


Cheers,
Flo


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