linux-kernel - Re: [PATCH 6/8] MIPS: call set_cpu_online() on the uping cpu with irq disabled

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Date:	Mon, 28 May 2012 17:34:51 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Yong Zhang <yong.zhang0@...il.com>
CC:	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	ralf@...ux-mips.org, sshtylyov@...sta.com, david.daney@...ium.com,
	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, axboe@...nel.dk,
	jeremy.fitzhardinge@...rix.com, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 6/8] MIPS: call set_cpu_online() on the uping cpu with
 irq disabled

On 05/22/2012 11:51 AM, Yong Zhang wrote:

> On Mon, May 21, 2012 at 04:09:16PM +0530, Srivatsa S. Bhat wrote:
>> On 05/21/2012 11:30 AM, Yong Zhang wrote:
>>
>>> From: Yong Zhang <yong.zhang@...driver.com>
>>>
>>> To prevent a problem as commit 5fbd036b [sched: Cleanup cpu_active madness]
>>> and commit 2baab4e9 [sched: Fix select_fallback_rq() vs cpu_active/cpu_online]
>>> try to resolve, move set_cpu_online() to the brought up CPU and with irq
>>> disabled.
>>>
>>> Signed-off-by: Yong Zhang <yong.zhang0@...il.com>
>>> Acked-by: David Daney <david.daney@...ium.com>
>>> ---
>>>  arch/mips/kernel/smp.c |    4 ++--
>>>  1 files changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
>>> index 73a268a..042145f 100644
>>> --- a/arch/mips/kernel/smp.c
>>> +++ b/arch/mips/kernel/smp.c
>>> @@ -122,6 +122,8 @@ asmlinkage __cpuinit void start_secondary(void)
>>>
>>>  	notify_cpu_starting(cpu);
>>>
>>> +	set_cpu_online(cpu, true);
>>> +
>>
>>
>> You will also need to use ipi_call_lock/unlock() around this.
>> See how x86 does it. (MIPS also selects USE_GENERIC_SMP_HELPERS).
> 
> Hmm... But look at the comments in arch/x86/kernel/smpboot.c::start_secondary()
> 
> start_secondary()
> {
> 	...
>         /*   
>          * We need to hold call_lock, so there is no inconsistency
>          * between the time smp_call_function() determines number of
>          * IPI recipients, and the time when the determination is made
>          * for which cpus receive the IPI. Holding this
>          * lock helps us to not include this cpu in a currently in progress
>          * smp_call_function().
>          *
>          * We need to hold vector_lock so there the set of online cpus
>          * does not change while we are assigning vectors to cpus.  Holding
>          * this lock ensures we don't half assign or remove an irq from a cpu.
>          */
>         ipi_call_lock();
>         lock_vector_lock();
>         set_cpu_online(smp_processor_id(), true);
>         unlock_vector_lock();
>         ipi_call_unlock();
> 
> 	...
> }
> 
> which ipi_call_lock()/ipi_call_unlock() is to pretect race with concurrent 
> smp_call_function(), but it seems that is already broken, because
> 
> 1) The comments is alread there before we switch to generic smp helper(commit
>    3b16cf87), and at that time the comments is true because
>    smp_call_function_interrupt() doesn't test if a cpu should handle the
>    IPI interrupt.
>    But in the gereric smp helper, we have checked if a cpu should handle the IPI
>    in generic_smp_call_function_interrupt():
>    	if (!cpumask_test_cpu(cpu, data->cpumask))
> 		continue;
> 
> 2) call_function.lock used in smp_call_function_many() is just to protect
>    call_function.queue and &data->refs, cpu_online_mask is outside of the
>    lock. And I don't think it's necessary to protect cpu_online_mask,
>    because data->cpumask is pre-calculate and even if a cpu is brougt up
>    when calling arch_send_call_function_ipi_mask(), it's harmless because
>    validation test in generic_smp_call_function_interrupt() will take care
>    of it.
> 
> 3) For cpu down issue, stop_machine() will guarantee that no concurrent
>    smp_call_fuction() is processing.
> 
> So it seems ipi_call_lock()/ipi_call_unlock() is not needed and could be
> removed IMHO.
> Or am I missing something?
> 


No, I think you are right. Sorry for the delay in replying.
It indeed looks like we need not use ipi_call_lock/unlock() in CPU bringup
code..

However, it does make me wonder about this:
commit 3d4422332 introduced the generic ipi helpers, and reduced the scope
of call_function.lock and also added the check in
generic_smp_call_function_interrupt() to proceed only if the cpu is present
in data->cpumask.

Then, commit 3b16cf8748 converted x86 to the generic ipi helpers, but while
doing that, it explicitly retained ipi_call_lock/unlock(), which is kind of
surprising.. I guess it was a mistake rather than intentional.
 
Regards,
Srivatsa S. Bhat

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