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

[Boqun Feng <boqun.feng@gmail.com>]

On Mon, Jul 22, 2024 at 11:27:48AM -0700, Dennis Zhou wrote:
> Hello,
> 
> On Mon, Jul 22, 2024 at 11:03:00AM -0700, Boqun Feng wrote:
> > 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.
> > 
> 
> I think a better alternative is we can move it up into the lock under
> area_found. The value gets updated as part of pcpu_alloc_area() as the
> code above populates percpu memory that is already allocated.
> 

Not sure I followed what exactly you suggested here because I'm not
familiar with the logic, but a simpler version would be:


diff --git a/mm/percpu.c b/mm/percpu.c
index 20d91af8c033..fc54d27e5786 100644
--- a/mm/percpu.c
+++ b/mm/percpu.c
@@ -1891,8 +1891,10 @@ 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)
-		pcpu_schedule_balance_work();
+	scoped_guard(spinlock_irqsave, &pcpu_lock) {
+		if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
+			pcpu_schedule_balance_work();
+	}
 
 	/* clear the areas and return address relative to base address */
 	for_each_possible_cpu(cpu)

I.e. just locking while checking.

Regards,
Boqun

> We should probably annotate pcpu_update_empty_pages() with:
>     lockdep_assert_held(&pcpu_lock);
> 
> Thanks,
> Dennis
> 
> > 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 */
> >