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

[Oliver Sang <oliver.sang@intel.com>]

hi, Dennis Zhou,

On Mon, Jul 22, 2024 at 10:50:53PM -0700, Dennis Zhou wrote:
> On Mon, Jul 22, 2024 at 01:53:52PM -0700, Boqun Feng wrote:
> > 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:
> > 
> > 
> 
> I believe that's the only naked access of pcpu_nr_empty_pop_pages. So
> I was thinking this'll fix this problem.
> 
> I also don't know how to rerun this CI tho..

we could test this patch. what's the base? could we apply it directly upon
24e44cc22a?

BTW, our bot is not so clever so far to auto test fix patches, so this is kind
of manual effort. due to resource constraint, it will be hard for us to test
each patch (we saw several patches in this thread already) or test very fast.

> 
> ---
> diff --git a/mm/percpu.c b/mm/percpu.c
> index 20d91af8c033..325fb8412e90 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1864,6 +1864,10 @@ void __percpu *pcpu_alloc_noprof(size_t size, size_t align, bool reserved,
>  
>  area_found:
>  	pcpu_stats_area_alloc(chunk, size);
> +
> +	if (pcpu_nr_empty_pop_pages < PCPU_EMPTY_POP_PAGES_LOW)
> +		pcpu_schedule_balance_work();
> +
>  	spin_unlock_irqrestore(&pcpu_lock, flags);
>  
>  	/* populate if not all pages are already there */
> @@ -1891,9 +1895,6 @@ 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();
> -
>  	/* clear the areas and return address relative to base address */
>  	for_each_possible_cpu(cpu)
>  		memset((void *)pcpu_chunk_addr(chunk, cpu, 0) + off, 0, size);