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BUG: KCSAN: data-race in __blk_mq_sched_dispatch_requests / blk_mq_do_dispatch_sched

write to 0xffff88812985a410 of 8 bytes by task 3630 on cpu 0:
 INIT_LIST_HEAD include/linux/list.h:36 [inline]
 list_splice_init include/linux/list.h:483 [inline]
 __blk_mq_sched_dispatch_requests+0x16a/0x2a0 block/blk-mq-sched.c:304
 blk_mq_sched_dispatch_requests+0x9f/0x110 block/blk-mq-sched.c:356
 __blk_mq_run_hw_queue+0xee/0x1c0 block/blk-mq.c:1534
 __blk_mq_delay_run_hw_queue+0x199/0x340 block/blk-mq.c:1611
 blk_mq_run_hw_queue+0x231/0x260 block/blk-mq.c:1664
 blk_mq_run_hw_queues block/blk-mq.c:1682 [inline]
 blk_mq_requeue_work+0x423/0x490 block/blk-mq.c:816
 process_one_work+0x3e1/0x9a0 kernel/workqueue.c:2269
 worker_thread+0x665/0xbe0 kernel/workqueue.c:2415
 kthread+0x20d/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff88812985a410 of 8 bytes by task 13936 on cpu 1:
 list_empty_careful include/linux/list.h:319 [inline]
 __blk_mq_do_dispatch_sched block/blk-mq-sched.c:153 [inline]
 blk_mq_do_dispatch_sched+0x17c/0x610 block/blk-mq-sched.c:214
 __blk_mq_sched_dispatch_requests+0x1fd/0x2a0 block/blk-mq-sched.c:325
 blk_mq_sched_dispatch_requests+0x9f/0x110 block/blk-mq-sched.c:356
 __blk_mq_run_hw_queue+0xee/0x1c0 block/blk-mq.c:1534
 __blk_mq_delay_run_hw_queue+0x199/0x340 block/blk-mq.c:1611
 blk_mq_run_hw_queue+0x231/0x260 block/blk-mq.c:1664
 blk_mq_sched_insert_requests+0x14d/0x200 block/blk-mq-sched.c:610
 blk_mq_flush_plug_list+0x2f5/0x420 block/blk-mq.c:1934
 blk_flush_plug_list+0x235/0x260 block/blk-core.c:1868
 blk_finish_plug+0x44/0x60 block/blk-core.c:1885
 read_pages+0x3b5/0x700 mm/readahead.c:150
 page_cache_readahead_unbounded+0x41a/0x470 mm/readahead.c:244
 ondemand_readahead+0x4e4/0x6e0 mm/readahead.c:273
 page_cache_sync_readahead+0x178/0x1a0 mm/readahead.c:585
 generic_file_buffered_read+0x2ec/0x1f60 mm/filemap.c:2101
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2405
 ext4_file_read_iter+0x2d8/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1876 [inline]
 generic_file_splice_read+0x22b/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:870 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:950
 do_splice_direct+0xf2/0x170 fs/splice.c:1059
 do_sendfile+0x56a/0xba0 fs/read_write.c:1540
 __do_sys_sendfile64 fs/read_write.c:1601 [inline]
 __se_sys_sendfile64 fs/read_write.c:1587 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1587
 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: 1 PID: 13936 Comm: syz-executor.0 Not tainted 5.9.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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