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

[Dennis Zhou]

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.

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 */
>