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BUG: KCSAN: data-race in wbt_inflight_cb / wbt_wait

write to 0xffff88812a695540 of 8 bytes by task 32345 on cpu 1:
 wb_timestamp block/blk-wbt.c:89 [inline]
 wb_timestamp block/blk-wbt.c:83 [inline]
 wbt_wait+0x22a/0x260 block/blk-wbt.c:587
 __rq_qos_throttle+0x42/0x70 block/blk-rq-qos.c:72
 rq_qos_throttle block/blk-rq-qos.h:182 [inline]
 blk_mq_make_request+0x297/0xf60 block/blk-mq.c:1998
 generic_make_request block/blk-core.c:1075 [inline]
 generic_make_request+0x196/0x700 block/blk-core.c:1017
 submit_bio+0x8f/0x3a0 block/blk-core.c:1200
 ext4_mpage_readpages+0xf1a/0x14d0 fs/ext4/readpage.c:414
 ext4_readpages+0x8e/0xb0 fs/ext4/inode.c:3243
 read_pages+0xa2/0x360 mm/readahead.c:126
 __do_page_cache_readahead+0x358/0x380 mm/readahead.c:212
 ra_submit mm/internal.h:62 [inline]
 do_sync_mmap_readahead mm/filemap.c:2407 [inline]
 filemap_fault+0x1027/0x1410 mm/filemap.c:2493
 ext4_filemap_fault+0x5b/0x74 fs/ext4/inode.c:6033
 __do_fault+0xa8/0x1e0 mm/memory.c:3450
 do_cow_fault mm/memory.c:3875 [inline]
 do_fault mm/memory.c:3977 [inline]
 handle_pte_fault mm/memory.c:4215 [inline]
 __handle_mm_fault+0xf42/0x2da0 mm/memory.c:4345
 handle_mm_fault+0x21c/0x540 mm/memory.c:4382
 do_user_addr_fault arch/x86/mm/fault.c:1464 [inline]
 do_page_fault+0x48a/0xa96 arch/x86/mm/fault.c:1535
 page_fault+0x34/0x40 arch/x86/entry/entry_64.S:1203

read to 0xffff88812a695540 of 8 bytes by task 7045 on cpu 0:
 get_limit block/blk-wbt.c:482 [inline]
 wbt_inflight_cb+0x1a8/0x220 block/blk-wbt.c:503
 rq_qos_wait+0x26b/0x300 block/blk-rq-qos.c:266
 __wbt_wait block/blk-wbt.c:526 [inline]
 wbt_wait+0x15f/0x260 block/blk-wbt.c:591
 __rq_qos_throttle+0x42/0x70 block/blk-rq-qos.c:72
 rq_qos_throttle block/blk-rq-qos.h:182 [inline]
 blk_mq_make_request+0x297/0xf60 block/blk-mq.c:1998
 generic_make_request block/blk-core.c:1075 [inline]
 generic_make_request+0x196/0x700 block/blk-core.c:1017
 submit_bio+0x8f/0x3a0 block/blk-core.c:1200
 ext4_io_submit+0xdf/0x110 fs/ext4/page-io.c:382
 ext4_writepages+0x5bb/0x1e10 fs/ext4/inode.c:2727
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __writeback_single_inode+0xaa/0x910 fs/fs-writeback.c:1452
 writeback_sb_inodes+0x4c8/0xa10 fs/fs-writeback.c:1716
 __writeback_inodes_wb+0xe9/0x1b0 fs/fs-writeback.c:1785
 wb_writeback+0x557/0x6a0 fs/fs-writeback.c:1894
 wb_check_start_all fs/fs-writeback.c:2018 [inline]
 wb_do_writeback fs/fs-writeback.c:2044 [inline]
 wb_workfn+0x71b/0x970 fs/fs-writeback.c:2078
 process_one_work+0x424/0x930 kernel/workqueue.c:2268
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2414
 kthread+0x203/0x230 kernel/kthread.c:268
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7045 Comm: kworker/u4:2 Not tainted 5.7.0-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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