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BUG: KCSAN: data-race in __wb_update_bandwidth / percpu_counter_add_batch

read to 0xffff88812a306930 of 8 bytes by task 7661 on cpu 1:
 __wb_update_bandwidth+0x99/0x3c0 mm/page-writeback.c:1364
 wb_update_bandwidth+0x90/0xc0 mm/page-writeback.c:1399
 wb_writeback+0x21d/0x6a0 fs/fs-writeback.c:1897
 wb_do_writeback fs/fs-writeback.c:2037 [inline]
 wb_workfn+0x22e/0x970 fs/fs-writeback.c:2078
 process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

write to 0xffff88812a306930 of 8 bytes by task 7517 on cpu 0:
 percpu_counter_add_batch+0xca/0x150 lib/percpu_counter.c:91
 __add_wb_stat include/linux/backing-dev.h:73 [inline]
 inc_wb_stat include/linux/backing-dev.h:78 [inline]
 account_page_dirtied+0x224/0x4a0 mm/page-writeback.c:2428
 __set_page_dirty+0x8a/0x270 fs/buffer.c:583
 mark_buffer_dirty+0x27f/0x2c0 fs/buffer.c:1112
 __ext4_handle_dirty_metadata+0x284/0x350 fs/ext4/ext4_jbd2.c:298
 ext4_mb_mark_diskspace_used+0x619/0x9d0 fs/ext4/mballoc.c:3029
 ext4_mb_new_blocks+0x6fc/0x1de0 fs/ext4/mballoc.c:4556
 ext4_ext_map_blocks+0x1c1e/0x2330 fs/ext4/extents.c:4460
 ext4_map_blocks+0x23a/0xfa0 fs/ext4/inode.c:640
 mpage_map_one_extent fs/ext4/inode.c:2468 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2517 [inline]
 ext4_writepages+0x136e/0x2300 fs/ext4/inode.c:2869
 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:2269
 worker_thread+0xa0/0x800 kernel/workqueue.c:2415
 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: 0 PID: 7517 Comm: kworker/u4:1 Not tainted 5.4.0-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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