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BUG: KCSAN: data-race in wb_timer_fn / wbt_wait

write to 0xffff88821a237504 of 4 bytes by interrupt on cpu 1:
 calc_wb_limits block/blk-wbt.c:304 [inline]
 scale_up block/blk-wbt.c:313 [inline]
 wb_timer_fn+0x56a/0xa30 block/blk-wbt.c:374
 blk_stat_timer_fn+0x3f4/0x410 block/blk-stat.c:99
 call_timer_fn+0x30/0x2a0 kernel/time/timer.c:1416
 expire_timers+0x116/0x290 kernel/time/timer.c:1461
 __run_timers+0x443/0x500 kernel/time/timer.c:1792
 run_timer_softirq+0x2e/0x60 kernel/time/timer.c:1805
 __do_softirq+0x198/0x360 kernel/softirq.c:298
 asm_call_on_stack+0xf/0x20 arch/x86/entry/entry_64.S:711
 __run_on_irqstack arch/x86/include/asm/irq_stack.h:22 [inline]
 run_on_irqstack_cond arch/x86/include/asm/irq_stack.h:48 [inline]
 do_softirq_own_stack+0x5d/0x80 arch/x86/kernel/irq_64.c:77
 invoke_softirq kernel/softirq.c:393 [inline]
 __irq_exit_rcu+0x115/0x120 kernel/softirq.c:423
 sysvec_apic_timer_interrupt+0xba/0xd0 arch/x86/kernel/apic/apic.c:1091
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:592
 is_atomic kernel/kcsan/core.c:229 [inline]
 should_watch kernel/kcsan/core.c:257 [inline]
 check_access kernel/kcsan/core.c:603 [inline]
 __tsan_read8+0x11a/0x180 kernel/kcsan/core.c:801
 __x64_sys_nanosleep+0x21/0x40 kernel/time/hrtimer.c:1953
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88821a237504 of 4 bytes by task 13707 on cpu 0:
 rwb_enabled block/blk-wbt.c:80 [inline]
 bio_to_wbt_flags block/blk-wbt.c:543 [inline]
 wbt_wait+0x36/0x2b0 block/blk-wbt.c:576
 __rq_qos_throttle+0x39/0x70 block/blk-rq-qos.c:72
 rq_qos_throttle block/blk-rq-qos.h:182 [inline]
 blk_mq_submit_bio+0x244/0x1130 block/blk-mq.c:2157
 __submit_bio_noacct_mq block/blk-core.c:1179 [inline]
 submit_bio_noacct+0x772/0x950 block/blk-core.c:1212
 submit_bio+0x200/0x370 block/blk-core.c:1282
 submit_bh_wbc+0x397/0x3d0 fs/buffer.c:3070
 submit_bh fs/buffer.c:3076 [inline]
 write_dirty_buffer+0xd4/0xe0 fs/buffer.c:3144
 fat_sync_bhs+0x52/0x160 fs/fat/misc.c:357
 fat_ent_write+0x85/0xd0 fs/fat/fatent.c:417
 fat_chain_add+0x149/0x400 fs/fat/misc.c:130
 fat_add_cluster fs/fat/inode.c:113 [inline]
 __fat_get_block fs/fat/inode.c:155 [inline]
 fat_get_block+0x429/0x5a0 fs/fat/inode.c:190
 __block_write_begin_int+0x4bf/0xfd0 fs/buffer.c:2011
 __block_write_begin fs/buffer.c:2061 [inline]
 block_write_begin+0x4f/0x190 fs/buffer.c:2120
 cont_write_begin+0x42c/0x610 fs/buffer.c:2469
 fat_write_begin+0x61/0xf0 fs/fat/inode.c:235
 generic_perform_write+0x196/0x390 mm/filemap.c:3388
 __generic_file_write_iter+0x154/0x330 mm/filemap.c:3517
 generic_file_write_iter+0x2f3/0x3e0 mm/filemap.c:3549
 call_write_iter include/linux/fs.h:1868 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x608/0x690 fs/read_write.c:578
 ksys_write+0xce/0x180 fs/read_write.c:631
 __do_sys_write fs/read_write.c:643 [inline]
 __se_sys_write fs/read_write.c:640 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:640
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 13707 Comm: syz-executor.3 Not tainted 5.8.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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