Re: [linus:master] [mm] 24e44cc22a: BUG:KCSAN:data-race_in_pcpu_alloc_noprof/pcpu_block_update_hint_alloc

[Boqun Feng]

On Mon, Jul 22, 2024 at 07:52:22AM -1000, Tejun Heo wrote:
> On Mon, Jul 22, 2024 at 10:47:30AM -0700, Boqun Feng wrote:
> > This looks like a data race because we read pcpu_nr_empty_pop_pages out
> > of the lock for a best effort checking, @Tejun, maybe you could confirm
> > on this?
> 
> That does sound plausible.
> 
> > -       if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
> > +       /*
> > +        * Checks pcpu_nr_empty_pop_pages out of the pcpu_lock, data races may
> > +        * occur but this is just a best-effort checking, everything is synced
> > +        * in pcpu_balance_work.
> > +        */
> > +       if (data_race(pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW)
> >                 pcpu_schedule_balance_work();
> 
> Would it be better to use READ/WRITE_ONCE() for the variable?
> 

For READ/WRITE_ONCE(), we will need to replace all write accesses and
all out-of-lock read accesses to pcpu_nr_empty_pop_pages, like below.
It's better in the sense that it doesn't rely on compiler behaviors on
data races, not sure about the performance impact though.

Regards,
Boqun

----->8
diff --git a/mm/percpu.c b/mm/percpu.c
index 20d91af8c033..729e8188238b 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -570,7 +570,8 @@ 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,
+			   pcpu_nr_empty_pop_pages - chunk->nr_empty_pop_pages);
 	}
 	list_move(&chunk->list, &pcpu_chunk_lists[pcpu_to_depopulate_slot]);
 }
@@ -581,7 +582,8 @@ 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,
+			   pcpu_nr_empty_pop_pages + chunk->nr_empty_pop_pages);
 		pcpu_chunk_relocate(chunk, -1);
 	}
 }
@@ -599,7 +601,8 @@ static inline void pcpu_update_empty_pages(struct pcpu_chunk *chunk, int nr)
 {
 	chunk->nr_empty_pop_pages += nr;
 	if (chunk != pcpu_reserved_chunk && !chunk->isolated)
-		pcpu_nr_empty_pop_pages += nr;
+		WRITE_ONCE(pcpu_nr_empty_pop_pages,
+			   pcpu_nr_empty_pop_pages + nr);
 }
 
 /*
@@ -1891,7 +1894,7 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
 		mutex_unlock(&pcpu_alloc_mutex);
 	}
 
-	if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
+	if (READ_ONCE(pcpu_nr_empty_pop_pages) < PCPU_EMPTY_POP_PAGES_LOW)
 		pcpu_schedule_balance_work();
 
 	/* clear the areas and return address relative to base address */
@@ -2754,7 +2757,7 @@ void __init pcpu_setup_first_chunk(const struct pcpu_alloc_info *ai,
 	tmp_addr = (unsigned long)base_addr + static_size + ai->reserved_size;
 	pcpu_first_chunk = pcpu_alloc_first_chunk(tmp_addr, dyn_size);
 
-	pcpu_nr_empty_pop_pages = pcpu_first_chunk->nr_empty_pop_pages;
+	WRITE_ONCE(pcpu_nr_empty_pop_pages, pcpu_first_chunk->nr_empty_pop_pages);
 	pcpu_chunk_relocate(pcpu_first_chunk, -1);
 
 	/* include all regions of the first chunk */