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

[Oleg Nesterov <oleg@redhat.com>]

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.

Oleg.