On 12/05/2012 03:47 AM, Andrew Morton wrote:

On Tue, 04 Dec 2012 14:24:28 +0530
"Srivatsa S. Bhat" [off-list ref] wrote:

quoted

From: Michael Wang <redacted>

With stop_machine() gone from the CPU offline path, we can't depend on
preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_stable_atomic() APIs to prevent CPUs from going
offline, while invoking from atomic context.

...

 	 */
-	this_cpu = get_cpu();
+	get_online_cpus_stable_atomic();
+	this_cpu = smp_processor_id();

I wonder if get_online_cpus_stable_atomic() should return the local CPU
ID.  Just as a little convenience thing.  Time will tell.

With the new version which doesn't use extra cpumasks, we won't have to
bother about this..
 

quoted

 	/*
 	 * Can deadlock when called with interrupts disabled.

...

@@ -380,15 +383,15 @@ int smp_call_function_any(const struct cpumask *mask,
 	nodemask = cpumask_of_node(cpu_to_node(cpu));
 	for (cpu = cpumask_first_and(nodemask, mask); cpu < nr_cpu_ids;
 	     cpu = cpumask_next_and(cpu, nodemask, mask)) {
-		if (cpu_online(cpu))
+		if (cpu_online_stable(cpu))
 			goto call;
 	}
 
 	/* Any online will do: smp_call_function_single handles nr_cpu_ids. */
-	cpu = cpumask_any_and(mask, cpu_online_mask);
+	cpu = cpumask_any_and(mask, cpu_online_stable_mask);
 call:
 	ret = smp_call_function_single(cpu, func, info, wait);
-	put_cpu();
+	put_online_cpus_stable_atomic();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(smp_call_function_any);

So smp_call_function_any() has no synchronization against CPUs coming
online.  Hence callers of smp_call_function_any() are responsible for
ensuring that CPUs which are concurrently coming online will adopt the
required state?

Yes.
 

I guess that has always been the case...

Right.

Regards,
Srivatsa S. Bhat