Re: [PATCH v1 1/1] xarray: fix the data-race in xas_find_chunk() by using READ_ONCE()

[Jan Kara]

On Mon 18-09-23 06:47:40, Mirsad Goran Todorovac wrote:
> KCSAN has discovered the following data-race:
> 
> [  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)
> [  206.510128] folio_end_writeback (mm/filemap.c:1616)
> [  206.510143] end_page_writeback (mm/folio-compat.c:28)
> [  206.510155] btrfs_page_clear_writeback (fs/btrfs/subpage.c:646) btrfs
> [  206.510994] end_bio_extent_writepage (./include/linux/bio.h:84 fs/btrfs/extent_io.c:542) btrfs
> [  206.511817] __btrfs_bio_end_io (fs/btrfs/bio.c:117 fs/btrfs/bio.c:112) btrfs
> [  206.512640] btrfs_orig_bbio_end_io (fs/btrfs/bio.c:164) btrfs
> [  206.513497] btrfs_simple_end_io (fs/btrfs/bio.c:380) btrfs
> [  206.514350] bio_endio (block/bio.c:1617)
> [  206.514362] blk_mq_end_request_batch (block/blk-mq.c:837 block/blk-mq.c:1073)
> [  206.514377] nvme_pci_complete_batch (drivers/nvme/host/pci.c:986) nvme
> [  206.514437] nvme_irq (drivers/nvme/host/pci.c:1086) nvme
> [  206.514500] __handle_irq_event_percpu (kernel/irq/handle.c:158)
> [  206.514517] handle_irq_event (kernel/irq/handle.c:195 kernel/irq/handle.c:210)
> [  206.514533] handle_edge_irq (kernel/irq/chip.c:836)
> [  206.514549] __common_interrupt (./include/linux/irqdesc.h:161 arch/x86/kernel/irq.c:238 arch/x86/kernel/irq.c:257)
> [  206.514563] common_interrupt (arch/x86/kernel/irq.c:247 (discriminator 14))
> [  206.514583] asm_common_interrupt (./arch/x86/include/asm/idtentry.h:636)
> [  206.514599] kcsan_setup_watchpoint (kernel/kcsan/core.c:705 (discriminator 1))
> [  206.514612] __tsan_read8 (kernel/kcsan/core.c:1025)
> [  206.514626] steal_from_bitmap.part.0 (./include/linux/find.h:186 fs/btrfs/free-space-cache.c:2557 fs/btrfs/free-space-cache.c:2613) btrfs
> [  206.515491] __btrfs_add_free_space (fs/btrfs/free-space-cache.c:2689 fs/btrfs/free-space-cache.c:2667) btrfs
> [  206.516361] btrfs_add_free_space_async_trimmed (fs/btrfs/free-space-cache.c:2798) btrfs
> [  206.517231] add_new_free_space (fs/btrfs/block-group.c:550) btrfs
> [  206.518095] load_free_space_tree (fs/btrfs/free-space-tree.c:1595 fs/btrfs/free-space-tree.c:1658) btrfs
> [  206.518953] caching_thread (fs/btrfs/block-group.c:873) btrfs
> [  206.519800] btrfs_work_helper (fs/btrfs/async-thread.c:314) btrfs
> [  206.520643] process_one_work (kernel/workqueue.c:2600)
> [  206.520658] worker_thread (./include/linux/list.h:292 kernel/workqueue.c:2752)
> [  206.520672] kthread (kernel/kthread.c:389)
> [  206.520684] ret_from_fork (arch/x86/kernel/process.c:145)
> [  206.520701] ret_from_fork_asm (arch/x86/entry/entry_64.S:312)
> 
> [  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)
> [  206.520763] __filemap_fdatawait_range (mm/filemap.c:519)
> [  206.520777] filemap_fdatawait_range (mm/filemap.c:556)
> [  206.520790] btrfs_wait_ordered_range (fs/btrfs/ordered-data.c:839) btrfs
> [  206.521641] btrfs_sync_file (fs/btrfs/file.c:1859) btrfs
> [  206.522495] vfs_fsync_range (fs/sync.c:188)
> [  206.522509] __x64_sys_fsync (./include/linux/file.h:45 fs/sync.c:213 fs/sync.c:220 fs/sync.c:218 fs/sync.c:218)
> [  206.522522] do_syscall_64 (arch/x86/entry/common.c:50 arch/x86/entry/common.c:80)
> [  206.522535] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
> 
> [  206.522557] value changed: 0xfffffffffff80000 -> 0xfffffffffff00000
> 
> [  206.522574] Reported by Kernel Concurrency Sanitizer on:
> [  206.522585] CPU: 6 PID: 2793 Comm: tracker-extract Tainted: G             L     6.5.0-rc6+ #44
> [  206.522600] Hardware name: ASRock X670E PG Lightning/X670E PG Lightning, BIOS 1.21 04/26/2023
> [  206.522608] ==================================================================

Thanks for working on this. I guess the full KCSAN warning isn't that
useful in the changelog. Rather I'd spend more time explaining the real
problem here ...

> As Jan Kara explained, the problem is in the function xas_find_chuck():
> 
> /* Private */
> static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
> 		xa_mark_t mark)
> {
> 	unsigned long *addr = xas->xa_node->marks[(__force unsigned)mark];
> 	unsigned int offset = xas->xa_offset;
> 
> 	if (advance)
> 		offset++;
> 	if (XA_CHUNK_SIZE == BITS_PER_LONG) {
> 		if (offset < XA_CHUNK_SIZE) {
> →			unsigned long data = *addr & (~0UL << offset);
> 			if (data)
> 				return __ffs(data);

... which is that xas_find_chunk() is called only under RCU protection and
thus the two uses of 'data' in the above code can yield different results.

> 		}
> 		return XA_CHUNK_SIZE;
> 	}
> 
> 	return find_next_bit(addr, XA_CHUNK_SIZE, offset);
> }
> 
> In particular, the line
> 
> 			unsigned long data = *addr & (~0UL << offset);
> 
> contains a data race that is best avoided using READ_ONCE(), which eliminated the KCSAN
> data-race warning completely.

Yes, this improves the situation for xarray use on 64-bit architectures but
doesn't fix cases on 32-bit archs or if CONFIG_BASE_SMALL is set. As I
mentioned in my previous reply, I'd rather:

1) Fix find_next_bit(), find_first_bit() and related functions in
lib/find_bit.c to use READ_ONCE() - such as _find_first_bit() etc. It is
quite some churn but I don't see how else to make these functions safe when
the underlying contents can change.

2) Change xas_find_chunk() to unconditionally use find_next_bit() as the
special case XA_CHUNK_SIZE == BITS_PER_LONG seems pointless these days
because find_next_bit() is inline and does small_const_nbits(size) check.

								Honza

 
> Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>
> Suggested-by: Jan Kara <jack@xxxxxxx>
> Fixes: b803b42823d0d ("xarray: Add XArray iterators")
> Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: Chris Mason <clm@xxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Josef Bacik <josef@xxxxxxxxxxxxxx>
> Cc: David Sterba <dsterba@xxxxxxxx>
> Cc: linux-btrfs@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@xxxxxxxxxxxx>
> ---
> v1: the proposed fix (RFC)
> 
>  include/linux/xarray.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/xarray.h b/include/linux/xarray.h
> index cb571dfcf4b1..1715fd322d62 100644
> --- a/include/linux/xarray.h
> +++ b/include/linux/xarray.h
> @@ -1720,7 +1720,7 @@ static inline unsigned int xas_find_chunk(struct xa_state *xas, bool advance,
>  		offset++;
>  	if (XA_CHUNK_SIZE == BITS_PER_LONG) {
>  		if (offset < XA_CHUNK_SIZE) {
> -			unsigned long data = *addr & (~0UL << offset);
> +			unsigned long data = READ_ONCE(*addr) & (~0UL << offset);
>  			if (data)
>  				return __ffs(data);
>  		}
> -- 
> 2.34.1
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR