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BUG: KCSAN: data-race in fprop_fraction_percpu / percpu_counter_add_batch

read to 0xffff8880a69faa40 of 8 bytes by task 28578 on cpu 0:
 fprop_fraction_percpu+0x8b/0x1c0 lib/flex_proportions.c:237
 __wb_calc_thresh+0x6f/0x250 mm/page-writeback.c:775
 wb_dirty_limits mm/page-writeback.c:1531 [inline]
 balance_dirty_pages+0x1187/0x1ba0 mm/page-writeback.c:1693
 balance_dirty_pages_ratelimited+0x703/0xa30 mm/page-writeback.c:1913
 generic_perform_write+0x22d/0x320 mm/filemap.c:3345
 ext4_buffered_write_iter+0x143/0x290 fs/ext4/file.c:252
 ext4_file_write_iter+0xf4/0xd40 fs/ext4/file.c:547
 call_write_iter include/linux/fs.h:1902 [inline]
 new_sync_write+0x388/0x4a0 fs/read_write.c:483
 __vfs_write+0xb1/0xc0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff8880a69faa40 of 8 bytes by interrupt on cpu 1:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 __fprop_inc_percpu_max+0x9a/0x150 lib/flex_proportions.c:271
 wb_domain_writeout_inc mm/page-writeback.c:581 [inline]
 __wb_writeout_inc mm/page-writeback.c:610 [inline]
 test_clear_page_writeback+0x5ca/0x790 mm/page-writeback.c:2737
 end_page_writeback+0x9e/0x190 mm/filemap.c:1339
 ext4_finish_bio+0x40d/0x520 fs/ext4/page-io.c:148
 ext4_end_bio+0xd2/0x2f0 fs/ext4/page-io.c:370
 bio_endio+0x2fa/0x400 block/bio.c:1821
 req_bio_endio block/blk-core.c:245 [inline]
 blk_update_request+0x427/0x7b0 block/blk-core.c:1464
 scsi_end_request+0x6b/0x360 drivers/scsi/scsi_lib.c:576
 scsi_io_completion+0x11d/0xc80 drivers/scsi/scsi_lib.c:960
 scsi_finish_command+0x283/0x380 drivers/scsi/scsi.c:228
 scsi_softirq_done+0x259/0x280 drivers/scsi/scsi_lib.c:1476
 blk_done_softirq+0x1eb/0x250 block/blk-softirq.c:37
 __do_softirq+0x115/0x33f kernel/softirq.c:292
 invoke_softirq kernel/softirq.c:373 [inline]
 irq_exit+0xbb/0xe0 kernel/softirq.c:413
 exiting_irq arch/x86/include/asm/apic.h:536 [inline]
 do_IRQ+0x81/0x130 arch/x86/kernel/irq.c:263
 ret_from_intr+0x0/0x21
 matching_access kernel/kcsan/encoding.h:90 [inline]
 find_watchpoint kernel/kcsan/core.c:105 [inline]
 check_access kernel/kcsan/core.c:446 [inline]
 __tsan_read8+0x31/0x100 kernel/kcsan/core.c:589
 mpage_process_page_bufs+0x40/0x400 fs/ext4/inode.c:2167
 mpage_prepare_extent_to_map+0x48b/0x760 fs/ext4/inode.c:2596
 ext4_writepages+0xb72/0x2500 fs/ext4/inode.c:2768
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __writeback_single_inode+0xaf/0x8e0 fs/fs-writeback.c:1452
 writeback_sb_inodes+0x4ba/0xa50 fs/fs-writeback.c:1716
 __writeback_inodes_wb+0xe8/0x1b0 fs/fs-writeback.c:1785
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1894
 wb_check_background_flush fs/fs-writeback.c:1962 [inline]
 wb_do_writeback fs/fs-writeback.c:2050 [inline]
 wb_workfn+0x696/0x970 fs/fs-writeback.c:2078
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2264
 worker_thread+0xa0/0x800 kernel/workqueue.c:2410
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 26539 Comm: kworker/u4:12 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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