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BUG: KCSAN: data-race in kthread_queue_work / kthread_worker_fn

write to 0xffff888102535d40 of 8 bytes by task 20170 on cpu 1:
 kthread_worker_fn+0x70/0x3a0 kernel/kthread.c:680
 loop_kthread_worker_fn+0x31/0x40 drivers/block/loop.c:937
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff888102535d40 of 8 bytes by task 20038 on cpu 0:
 kthread_insert_work kernel/kthread.c:839 [inline]
 kthread_queue_work+0x14c/0x1a0 kernel/kthread.c:863
 loop_queue_rq+0xf4/0x230 drivers/block/loop.c:2031
 blk_mq_dispatch_rq_list+0x4cc/0xd30 block/blk-mq.c:1388
 __blk_mq_do_dispatch_sched block/blk-mq-sched.c:186 [inline]
 blk_mq_do_dispatch_sched+0x370/0x610 block/blk-mq-sched.c:199
 __blk_mq_sched_dispatch_requests+0x1fd/0x2a0 block/blk-mq-sched.c:310
 blk_mq_sched_dispatch_requests+0x8f/0xf0 block/blk-mq-sched.c:341
 __blk_mq_run_hw_queue+0xee/0x1c0 block/blk-mq.c:1532
 __blk_mq_delay_run_hw_queue+0x17d/0x330 block/blk-mq.c:1609
 blk_mq_run_hw_queue+0x231/0x260 block/blk-mq.c:1662
 blk_mq_sched_insert_requests+0x144/0x210 block/blk-mq-sched.c:501
 blk_mq_flush_plug_list+0x2f5/0x400 block/blk-mq.c:1932
 blk_flush_plug_list+0x235/0x260 block/blk-core.c:1756
 blk_finish_plug+0x44/0x60 block/blk-core.c:1773
 read_pages+0x3a6/0x6e0 mm/readahead.c:150
 page_cache_ra_unbounded+0x464/0x4c0 mm/readahead.c:238
 do_page_cache_ra mm/readahead.c:267 [inline]
 force_page_cache_ra+0x226/0x260 mm/readahead.c:298
 page_cache_sync_ra+0x194/0x1c0 mm/readahead.c:572
 page_cache_sync_readahead include/linux/pagemap.h:839 [inline]
 generic_file_buffered_read+0x38d/0x2110 mm/filemap.c:2227
 generic_file_read_iter+0x80/0x3d0 mm/filemap.c:2535
 blkdev_read_iter+0xb3/0xc0 fs/block_dev.c:1925
 call_read_iter include/linux/fs.h:1881 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x56d/0x5e0 fs/read_write.c:481
 ksys_read+0xce/0x180 fs/read_write.c:615
 __do_sys_read fs/read_write.c:625 [inline]
 __se_sys_read fs/read_write.c:623 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:623
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 20038 Comm: systemd-udevd Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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