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

[Oliver Sang]

hi, Dannis,

On Mon, Jul 22, 2024 at 11:13:10PM -0700, Dennis Zhou wrote:
> Hi Oliver,
> 
> On Tue, Jul 23, 2024 at 02:09:38PM +0800, Oliver Sang wrote:
> > 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.
> > 
> 
> Ah yeah that makes sense. If you don't mind testing the last one I sent,
> the one below, that applies cleanly to 24e44cc22a.

in our tests, you patch could solve the KCSAN issues. thanks

=========================================================================================
compiler/group/kconfig/nr_groups/rootfs/runtime/tbox_group/testcase:
  gcc-13/group-04/x86_64-randconfig-013-20240713/5/debian-11.1-i386-20220923.cgz/300s/vm-snb/trinity

commit:
  60fa4a9e23231 ("mm: percpu: add codetag reference into pcpuobj_ext")
  24e44cc22aa31 ("mm: percpu: enable per-cpu allocation tagging")
  dcfbb68202759 <--- your patch

60fa4a9e23231721 24e44cc22aa3112082f2ee23137 dcfbb6820275994e92a9dcf309e
---------------- --------------------------- ---------------------------
       fail:runs  %reproduction    fail:runs  %reproduction    fail:runs
           |             |             |             |             |
        281:999        -28%            :998        -28%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc/pcpu_block_update_hint_alloc
        296:999        -30%            :998        -30%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc/pcpu_block_update_hint_free
         25:999         -3%            :998         -3%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc/pcpu_chunk_populated
           :999         29%         292:998          0%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc_noprof/pcpu_block_update_hint_alloc
           :999         27%         269:998          0%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc_noprof/pcpu_block_update_hint_free
           :999          4%          44:998          0%            :999   dmesg.BUG:KCSAN:data-race_in_pcpu_alloc_noprof/pcpu_chunk_populated

> 
> Thanks,
> Dennis
> 
> > > 
> > > ---
> > > 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);