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

[Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>]

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] ==================================================================

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);
		}
		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.

Reported-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Suggested-by: Jan Kara <jack@suse.cz>
Fixes: b803b42823d0d ("xarray: Add XArray iterators")
Matthew Wilcox <willy@infradead.org>
Cc: Chris Mason <clm@fb.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-mm@kvack.org
Signed-off-by: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
---
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