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BUG: KCSAN: data-race in blk_mq_free_request / blk_mq_free_request

write to 0xffffe8ffffc34808 of 8 bytes by interrupt on cpu 0:
 blk_mq_free_request+0x17d/0x330 block/blk-mq.c:522
 __blk_mq_end_request+0x214/0x230 block/blk-mq.c:557
 blk_mq_end_request+0x37/0x50 block/blk-mq.c:566
 end_cmd+0x80/0x140 drivers/block/null_blk/main.c:699
 null_complete_rq+0x18/0x20 drivers/block/null_blk/main.c:726
 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 0xffffe8ffffc34808 of 8 bytes by interrupt on cpu 1:
 blk_mq_free_request+0x16a/0x330 block/blk-mq.c:522
 __blk_mq_end_request+0x214/0x230 block/blk-mq.c:557
 blk_mq_end_request+0x37/0x50 block/blk-mq.c:566
 end_cmd+0x80/0x140 drivers/block/null_blk/main.c:699
 null_complete_rq+0x18/0x20 drivers/block/null_blk/main.c:726
 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

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18 Comm: ksoftirqd/1 Not tainted 5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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