Hello,

On Mon, Oct 25, 2021 at 09:50:48AM +0200, Christoph Lameter wrote:

On Mon, 25 Oct 2021, Yuanzheng Song wrote:

quoted

When reading the pcpu_nr_empty_pop_pages in pcpu_alloc()
and writing the pcpu_nr_empty_pop_pages in
pcpu_update_empty_pages() at the same time,
the data-race occurs.

Looks like a use case for the atomic RMV instructions.

Yeah. I see 2 options. Switch the variable over to an atomic or we can
move the read behind pcpu_lock. All the writes are already behind it
othewise that would actually be problematic. In this particular case,
reading a wrong # of empty pages isn't a big deal as eventually the
background work will get scheduled.

Thanks,
Dennis

quoted

To fix this issue, use READ_ONCE() and WRITE_ONCE() to
read and write the pcpu_nr_empty_pop_pages.

Never thought that READ_ONCE and WRITE_ONCE can fix races like
this. Really?

quoted

diff --git a/mm/percpu.c b/mm/percpu.c
index 293009cc03ef..e8ef92e698ab 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c

@@ -574,7 +574,9 @@ static void pcpu_isolate_chunk(struct pcpu_chunk *chunk)

 	if (!chunk->isolated) {
 		chunk->isolated = true;
-		pcpu_nr_empty_pop_pages -= chunk->nr_empty_pop_pages;
+		WRITE_ONCE(pcpu_nr_empty_pop_pages,
+			   READ_ONCE(pcpu_nr_empty_pop_pages) -
+			   chunk->nr_empty_pop_pages);

atomic_sub()?

quoted

 	}
 	list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]);
 }

@@ -585,7 +587,9 @@ static void pcpu_reintegrate_chunk(struct pcpu_chunk *chunk)

 	if (chunk->isolated) {
 		chunk->isolated = false;
-		pcpu_nr_empty_pop_pages += chunk->nr_empty_pop_pages;
+		WRITE_ONCE(pcpu_nr_empty_pop_pages,
+			   READ_ONCE(pcpu_nr_empty_pop_pages) +
+			   chunk->nr_empty_pop_pages);

atomic_add()?