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Date: Wed, 4 Aug 2010 20:21:15 +0100
From: Matthew Garrett <mjg59@srcf.ucam.org>
To: david@lang.hm
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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Subject: Re: Attempted summary of suspend-blockers LKML thread
Message-ID: <20100804192115.GA2946@srcf.ucam.org>
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On Wed, Aug 04, 2010 at 12:15:59PM -0700, david@lang.hm wrote:
> On Wed, 4 Aug 2010, Matthew Garrett wrote:
>> No! And that's precisely the issue. Android's existing behaviour could
>> be entirely implemented in the form of binary that manually triggers
>> suspend when (a) the screen is off and (b) no userspace applications
>> have indicated that the system shouldn't sleep, except for the wakeup
>> event race. Imagine the following:
>>
>> 1) The policy timeout is about to expire. No applications are holding
>> wakelocks. The system will suspend providing nothing takes a wakelock.
>> 2) A network packet arrives indicating an incoming SIP call
>> 3) The VOIP application takes a wakelock and prevents the phone from
>> suspending while the call is in progress
>>
>> What stops the system going to sleep between (2) and (3)? cgroups don't,
>> because the voip app is an otherwise untrusted application that you've
>> just told the scheduler to ignore.
>
> Even in the current implementation (wakelocks), Since the VOIP 
> application isn't allowed to take a wakelock, wouldn't the system go to 
> sleep immediatly anyway, even if the application gets the packet and 
> starts the call? What would ever raise the wakelock to keep the phone 
> from sleeping in the middle of the call?

There's two parts of that. The first is that the voip application is 
allowed to take a wakelock - but that doesn't mean that you trust it the 
rest of the time.The second is that the incoming network packet causes 
the kernel to take a wakelock that will be released once userspace has 
processed the network packet. This ensures that at least one wakelock is 
held for the entire relevant period of time.

-- 
Matthew Garrett | mjg59@srcf.ucam.org
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