Re: [BUG] KCSAN: data-race in xas_clear_mark / xas_find_marked

[Jan Kara]

On Fri 18-08-23 13:21:17, Matthew Wilcox wrote:
> On Fri, Aug 18, 2023 at 10:01:32AM +0200, Mirsad Todorovac wrote:
> > [  206.510010] ==================================================================
> > [  206.510035] BUG: KCSAN: data-race in xas_clear_mark / xas_find_marked
> > 
> > [  206.510067] write to 0xffff963df6a90fe0 of 8 bytes by interrupt on cpu 22:
> > [  206.510081]  xas_clear_mark+0xd5/0x180
> > [  206.510097]  __xa_clear_mark+0xd1/0x100
> > [  206.510114]  __folio_end_writeback+0x293/0x5a0
> > [  206.520722] read to 0xffff963df6a90fe0 of 8 bytes by task 2793 on cpu 6:
> > [  206.520735]  xas_find_marked+0xe5/0x600
> > [  206.520750]  filemap_get_folios_tag+0xf9/0x3d0
> Also, before submitting this kind of report, you should run the
> trace through scripts/decode_stacktrace.sh to give us line numbers
> instead of hex offsets, which are useless to anyone who doesn't have
> your exact kernel build.
> 
> > [  206.510010] ==================================================================
> > [  206.510035] BUG: KCSAN: data-race in xas_clear_mark / xas_find_marked
> > 
> > [  206.510067] write to 0xffff963df6a90fe0 of 8 bytes by interrupt on cpu 22:
> > [  206.510081] xas_clear_mark (./arch/x86/include/asm/bitops.h:178 ./include/asm-generic/bitops/instrumented-non-atomic.h:115 lib/xarray.c:102 lib/xarray.c:914)
> > [  206.510097] __xa_clear_mark (lib/xarray.c:1923)
> > [  206.510114] __folio_end_writeback (mm/page-writeback.c:2981)
> 
> This path is properly using xa_lock_irqsave() before calling
> __xa_clear_mark().
> 
> > [  206.520722] read to 0xffff963df6a90fe0 of 8 bytes by task 2793 on cpu 6:
> > [  206.520735] xas_find_marked (./include/linux/xarray.h:1706 lib/xarray.c:1354)
> > [  206.520750] filemap_get_folios_tag (mm/filemap.c:1975 mm/filemap.c:2273)
> 
> This takes the RCU read lock before calling xas_find_marked() as it's
> supposed to.
> 
> What garbage do I have to write to tell KCSAN it's wrong?  The line
> that's probably triggering it is currently:
> 
>                         unsigned long data = *addr & (~0UL << offset);

I don't think it is actually wrong in this case. You're accessing xarray
only with RCU protection so it can be changing under your hands. For
example the code in xas_find_chunk():

                        unsigned long data = *addr & (~0UL << offset);
                        if (data)
                                return __ffs(data);

is prone to the compiler refetching 'data' from *addr after checking for
data != 0 and getting 0 the second time which would trigger undefined
behavior of __ffs(). So that code should definitely use READ_ONCE() to make
things safe.

BTW, find_next_bit() seems to need a similar treatment and in fact I'm not
sure why xas_find_chunk() has a special case for XA_CHUNK_SIZE ==
BITS_PER_LONG because find_next_bit() checks for that and handles that in a
fast path in the same way.

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR