linux-kernel - Re: [PATCH 2/2] CPU hotplug, stop-machine: Plug race-window that leads to "IPI-to-offline-CPU"

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Message-ID: <536953C4.8030201@linux.vnet.ibm.com>
Date:	Wed, 07 May 2014 02:57:32 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>, peterz@...radead.org,
	tglx@...utronix.de, mingo@...nel.org, rusty@...tcorp.com.au,
	fweisbec@...il.com, hch@...radead.org, mgorman@...e.de,
	riel@...hat.com, bp@...e.de, rostedt@...dmis.org,
	mgalbraith@...e.de, ego@...ux.vnet.ibm.com,
	paulmck@...ux.vnet.ibm.com, oleg@...hat.com, rjw@...ysocki.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] CPU hotplug, stop-machine: Plug race-window that
 leads to "IPI-to-offline-CPU"

On 05/07/2014 02:12 AM, Tejun Heo wrote:
> On Tue, May 06, 2014 at 01:40:54PM -0700, Andrew Morton wrote:
>> On Tue, 06 May 2014 23:33:03 +0530 "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>>
>>> --- a/kernel/stop_machine.c
>>> +++ b/kernel/stop_machine.c
>>> @@ -165,12 +165,21 @@ static void ack_state(struct multi_stop_data *msdata)
>>>  		set_state(msdata, msdata->state + 1);
>>>  }
>>>  
>>> +/* Holding area for active CPUs, to let all the non-active CPUs go first */
>>> +static void hold_active_cpus(struct multi_stop_data *msdata,
>>> +			     int num_active_cpus)
>>> +{
>>> +	/* Wait until all the non-active threads ack the state */
>>> +	while (atomic_read(&msdata->thread_ack) > num_active_cpus)
>>> +		cpu_relax();
>>> +}
>>
>> The code comments are a bit lame.  Can we do a better job of explaining
>> the overall dynamic behaviour?  Help readers to understand the problem
>> which hold_active_cpus() is solving and how it solves it?
> 
> Does it even need to be a separate function?  I kinda really dislike
> trivial helpers which are used only once.  It obfuscates more than
> helping anything.  I think proper comment where the actual
> synchronization is happening along with open coded wait would be
> easier to follow.
> 

Ok, I'll open code it and add an appropriate comment explaining the
synchronization.

Thank you!
 
Regards,
Srivatsa S. Bhat

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