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BUG: KCSAN: data-race in __blk_mq_free_request / bt_for_each

write to 0xffff8881028b1a50 of 8 bytes by interrupt on cpu 0:
 __blk_mq_free_request+0x107/0x1c0 block/blk-mq.c:497
 blk_mq_free_request+0x2c8/0x330 block/blk-mq.c:533
 __blk_mq_end_request+0x214/0x230 block/blk-mq.c:557
 blk_mq_end_request+0x37/0x50 block/blk-mq.c:566
 lo_complete_rq+0xca/0x180 drivers/block/loop.c:497
 blk_complete_reqs block/blk-mq.c:576 [inline]
 blk_done_softirq+0x69/0x90 block/blk-mq.c:581
 __do_softirq+0x13c/0x2c3 kernel/softirq.c:345
 run_ksoftirqd+0x13/0x20 kernel/softirq.c:652
 smpboot_thread_fn+0x34f/0x520 kernel/smpboot.c:165
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

read to 0xffff8881028b1a50 of 8 bytes by task 2135 on cpu 1:
 bt_iter block/blk-mq-tag.c:218 [inline]
 __sbitmap_for_each_set include/linux/sbitmap.h:263 [inline]
 sbitmap_for_each_set include/linux/sbitmap.h:284 [inline]
 bt_for_each+0x239/0x350 block/blk-mq-tag.c:247
 blk_mq_queue_tag_busy_iter+0x19f/0x280 block/blk-mq-tag.c:436
 blk_mq_timeout_work+0x9a/0x2a0 block/blk-mq.c:976
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 2135 Comm: kworker/1:1H Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: kblockd blk_mq_timeout_work
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