Re: [RFC PATCH v4 1/9] CPU hotplug: Provide APIs to prevent CPU offline from atomic context

["Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>]

On 12/23/2012 10:12 PM, Oleg Nesterov wrote:
> On 12/23, Srivatsa S. Bhat wrote:
>>
>> On 12/20/2012 07:12 PM, Oleg Nesterov wrote:
>>>
>>> We need mb() + rmb(). Plust cli/sti unless this arch has optimized
>>> this_cpu_add() like x86 (as you pointed out).
>>>
>>
>> Hey, IIUC, we actually don't need mb() in the reader!! Just an rmb() will do.
> 
> Well. I don't think so. But when it comes to the barriers I am never sure
> until Paul confirms my understanding ;)
> 
>> #define reader_nested_percpu()						\
>> 	     (__this_cpu_read(reader_percpu_refcnt) & READER_REFCNT_MASK)
>>
>> #define writer_active()							\
>> 				(__this_cpu_read(writer_signal))
>>
>>
>> #define READER_PRESENT		(1UL << 16)
>> #define READER_REFCNT_MASK	(READER_PRESENT - 1)
>>
>> void get_online_cpus_atomic(void)
>> {
>> 	preempt_disable();
>>
>> 	/*
>> 	 * First and foremost, make your presence known to the writer.
>> 	 */
>> 	this_cpu_add(reader_percpu_refcnt, READER_PRESENT);
>>
>> 	/*
>> 	 * If we are already using per-cpu refcounts, it is not safe to switch
>> 	 * the synchronization scheme. So continue using the refcounts.
>> 	 */
>> 	if (reader_nested_percpu()) {
>> 		this_cpu_inc(reader_percpu_refcnt);
>> 	} else {
>> 		smp_rmb();
>> 		if (unlikely(writer_active())) {
>> 			... //take hotplug_rwlock
>> 		}
>> 	}
>>
>> 	...
>>
>> 	/* Prevent reordering of any subsequent reads of cpu_online_mask. */
>> 	smp_rmb();
>> }
>>
>> The smp_rmb() before writer_active() ensures that LOAD(writer_signal) follows
>> LOAD(reader_percpu_refcnt) (at the 'if' condition). And in turn, that load is
>> automatically going to follow the STORE(reader_percpu_refcnt)
> 
> But why this STORE should be visible on another CPU before we LOAD(writer_signal)?
> 
> Lets discuss the simple and artificial example. Suppose we have
> 
> 	int X, Y;
> 
> 	int func(void)
> 	{
> 		X = 1;	// suppose that nobody else can change it
> 		mb();
> 		return Y;
> 	}
> 
> Now you are saying that we can change it and avoid the costly mb():
> 
> 	int func(void)
> 	{
> 		X = 1;
> 
> 		if (X != 1)
> 			BUG();
> 	
> 		rmb();
> 		return Y;
> 	}
> 
> I doubt. rmb() can only guarantee that the preceding LOAD's should be
> completed. Without mb() it is possible that this CPU won't write X to
> memory at all.
> 

Oh, ok :-( Thanks for correcting me and for the detailed explanation!
For a moment, I really thought we had it solved at last! ;-(

Regards,
Srivatsa S. Bhat