On 09/06/2017 05:03 PM, Michael Bringmann wrote:

On 09/06/2017 09:45 AM, Nathan Fontenot wrote:

quoted

On 09/01/2017 10:48 AM, Michael Bringmann wrote:

quoted

powerpc/vphn: On Power systems with shared configurations of CPUs
and memory, there are some issues with the association of additional
CPUs and memory to nodes when hot-adding resources.  This patch
fixes an end-of-updates processing problem observed occasionally
in numa_update_cpu_topology().

Signed-off-by: Michael Bringmann <redacted>
---
 arch/powerpc/mm/numa.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/arch/powerpc/mm/numa.c b/arch/powerpc/mm/numa.c
index 3a5b334..fccf23f 100644
--- a/arch/powerpc/mm/numa.c
+++ b/arch/powerpc/mm/numa.c

@@ -1410,6 +1410,13 @@ int numa_update_cpu_topology(bool cpus_locked)
 		cpu = cpu_last_thread_sibling(cpu);
 	}

+	/*
+	 * Prevent processing of 'updates' from overflowing array
+	 * in cases where last entry filled in a 'next' pointer.
+	 */
+	if (i)
+		updates[i-1].next = NULL;
+

This really looks like the bug is in the code above this where we
fill in the updates array for each of the sibling cpus. The code
there assumes that if the current update entry is not the end that
there will be more updates and blindly sets the next pointer.

Perhaps correcting the logic in that code to next pointers. Set the
ud pointer to NULL before the outer for_each_cpu() loop. Then in the
inner for_each_cpu(sibling,...) loop update the ud-> next pointer as
the first operation.

		for_each_cpu(sibling, cpu_sibling_mask(cpu)) {
			if (ud)
				ud->next = &updates[i];
			...
		}

Obviously untested, but I think this would prevent setting the next
pointer in the last update entry that is filled out erroneously.

The above fragment looks to skip initialization of the 'next' pointer
in the first element of the the 'updates'.  That would abort subsequent
evaluation of the array too soon, I believe.  I would like to take another look
to see whether the current check 'if (i < weight) ud->next = &updates[i];'
is having problems due to i being 0-relative and weight being 1-relative.

Another thing to keep in mind is that cpus can be skipped by checks earlier
in the loop. There is not guarantee that we will add 'weight' elements to
the ud list.

-Nathan
 

quoted

  
-Nathan

Michael

quoted

quoted

 	pr_debug("Topology update for the following CPUs:\n");
 	if (cpumask_weight(&updated_cpus)) {
 		for (ud = &updates[0]; ud; ud = ud->next) {