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BUG: KCSAN: data-race in __xa_clear_mark / xas_find_marked

read-write to 0xffff88811a981228 of 8 bytes by interrupt on cpu 0:
 instrument_read_write include/linux/instrumented.h:56 [inline]
 __instrument_read_write_bitop include/asm-generic/bitops/instrumented-non-atomic.h:84 [inline]
 ___test_and_clear_bit include/asm-generic/bitops/instrumented-non-atomic.h:114 [inline]
 node_clear_mark lib/xarray.c:102 [inline]
 xas_clear_mark lib/xarray.c:922 [inline]
 __xa_clear_mark+0xf5/0x1e0 lib/xarray.c:2103
 __folio_end_writeback+0xe7/0x530 mm/page-writeback.c:2965
 folio_end_writeback_no_dropbehind+0x6d/0x200 mm/filemap.c:1672
 folio_end_writeback+0x1c/0x70 mm/filemap.c:1698
 bh_end_async_write+0x1a6/0x250 fs/buffer.c:432
 bio_endio+0x390/0x570 block/bio.c:1929
 blk_update_request+0x33a/0x710 block/blk-mq.c:1029
 blk_mq_end_request+0x26/0x50 block/blk-mq.c:1160
 lo_complete_rq+0x98/0x130 drivers/block/loop.c:315
 blk_complete_reqs block/blk-mq.c:1235 [inline]
 blk_done_softirq+0x77/0xb0 block/blk-mq.c:1240
 handle_softirqs+0xcc/0x2a0 kernel/softirq.c:645
 __do_softirq kernel/softirq.c:679 [inline]
 invoke_softirq kernel/softirq.c:519 [inline]
 __irq_exit_rcu+0x44/0xe0 kernel/softirq.c:758
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1062 [inline]
 sysvec_apic_timer_interrupt+0x74/0x80 arch/x86/kernel/apic/apic.c:1062
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:674
 __tsan_unaligned_volatile_write4+0x0/0x190 kernel/kcsan/core.c:761
 arch_atomic_set arch/x86/include/asm/atomic.h:28 [inline]
 raw_atomic_set include/linux/atomic/atomic-arch-fallback.h:503 [inline]
 atomic_set include/linux/atomic/atomic-instrumented.h:68 [inline]
 set_page_count include/linux/page_ref.h:117 [inline]
 set_page_refcounted mm/internal.h:664 [inline]
 folio_alloc_mpol_noprof+0x52/0x80 mm/mempolicy.c:2510
 shmem_alloc_folio mm/shmem.c:1936 [inline]
 shmem_alloc_and_add_folio mm/shmem.c:1978 [inline]
 shmem_get_folio_gfp+0x3ff/0xd90 mm/shmem.c:2531
 shmem_get_folio mm/shmem.c:2637 [inline]
 shmem_write_begin+0xfc/0x1f0 mm/shmem.c:3268
 generic_perform_write+0x186/0x490 mm/filemap.c:4374
 shmem_file_write_iter+0xc5/0xf0 mm/shmem.c:3443
 lo_rw_aio+0x6ab/0x760 drivers/block/loop.c:-1
 do_req_filebacked drivers/block/loop.c:-1 [inline]
 loop_handle_cmd drivers/block/loop.c:1949 [inline]
 loop_process_work+0x569/0xad0 drivers/block/loop.c:1984
 loop_workfn+0x31/0x40 drivers/block/loop.c:2008
 process_one_work kernel/workqueue.c:3396 [inline]
 process_scheduled_works+0x56a/0xa50 kernel/workqueue.c:3479
 worker_thread+0x569/0x750 kernel/workqueue.c:3560
 kthread+0x223/0x270 kernel/kthread.c:436
 ret_from_fork+0x146/0x330 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

read to 0xffff88811a981228 of 8 bytes by task 4619 on cpu 1:
 xas_find_chunk include/linux/xarray.h:1752 [inline]
 xas_find_marked+0x211/0x650 lib/xarray.c:1513
 find_get_entry mm/filemap.c:2087 [inline]
 filemap_get_folios_tag+0xf7/0x400 mm/filemap.c:2353
 __filemap_fdatawait_range mm/filemap.c:523 [inline]
 filemap_write_and_wait_range+0x1f5/0x370 mm/filemap.c:692
 filemap_write_and_wait include/linux/pagemap.h:65 [inline]
 sync_blockdev+0x35/0x50 block/bdev.c:275
 loop_set_status+0x110/0x670 drivers/block/loop.c:1265
 loop_set_status_old drivers/block/loop.c:1382 [inline]
 lo_ioctl+0x78f/0x1430 drivers/block/loop.c:1575
 blkdev_ioctl+0x387/0x460 block/ioctl.c:798
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:597 [inline]
 __se_sys_ioctl+0xce/0x140 fs/ioctl.c:583
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:583
 x64_sys_call+0x239d/0x2550 arch/x86/include/generated/asm/syscalls_64.h:17
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x112/0x360 arch/x86/entry/syscall_64.c:84
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

value changed: 0xfffff80000000000 -> 0xffc0000000000000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 UID: 0 PID: 4619 Comm: syz.1.268 Not tainted syzkaller #0 PREEMPT(lazy) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/16/2026
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loop7: detected capacity change from 16384 to 0