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BUG: KCSAN: data-race in wb_timer_fn / wbt_wait

write to 0xffff888103a33f04 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+0x42a/0xa00 block/blk-wbt.c:382
 blk_stat_timer_fn+0x3f4/0x410 block/blk-stat.c:99
 call_timer_fn+0x2e/0x240 kernel/time/timer.c:1431
 expire_timers+0x116/0x260 kernel/time/timer.c:1476
 __run_timers+0x358/0x3f0 kernel/time/timer.c:1745
 run_timer_softirq+0x19/0x30 kernel/time/timer.c:1758
 __do_softirq+0x13c/0x2c3 kernel/softirq.c:345
 invoke_softirq kernel/softirq.c:221 [inline]
 __irq_exit_rcu+0xb4/0xc0 kernel/softirq.c:422
 sysvec_apic_timer_interrupt+0x6e/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:632
 smap_restore arch/x86/include/asm/smap.h:70 [inline]
 kcsan_setup_watchpoint+0x436/0x470 kernel/kcsan/core.c:590
 find_inode_bit+0x2d9/0x3a0 fs/ext4/ialloc.c:731
 __ext4_new_inode+0xb6c/0x2fe0 fs/ext4/ialloc.c:1063
 ext4_symlink+0x2d1/0x780 fs/ext4/namei.c:3347
 vfs_symlink+0xbe/0x170 fs/namei.c:4176
 do_symlinkat+0x11e/0x2b0 fs/namei.c:4206
 __do_sys_symlink fs/namei.c:4227 [inline]
 __se_sys_symlink fs/namei.c:4225 [inline]
 __x64_sys_symlink+0x32/0x40 fs/namei.c:4225
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888103a33f04 of 4 bytes by task 21013 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+0x253/0x1060 block/blk-mq.c:2185
 __submit_bio_noacct_mq block/blk-core.c:1014 [inline]
 submit_bio_noacct+0x7c2/0x9a0 block/blk-core.c:1047
 submit_bio+0x200/0x370 block/blk-core.c:1118
 submit_bh_wbc+0x36b/0x3b0 fs/buffer.c:3055
 submit_bh fs/buffer.c:3061 [inline]
 __sync_dirty_buffer+0x136/0x1e0 fs/buffer.c:3156
 sync_dirty_buffer+0x16/0x20 fs/buffer.c:3169
 __ext4_handle_dirty_metadata+0x1d3/0x590 fs/ext4/ext4_jbd2.c:364
 ext4_handle_dirty_dirblock+0x184/0x330 fs/ext4/namei.c:407
 ext4_delete_entry+0x240/0x350 fs/ext4/namei.c:2533
 ext4_rmdir+0x5ff/0x970 fs/ext4/namei.c:3159
 vfs_rmdir+0x13b/0x2a0 fs/namei.c:3899
 do_rmdir+0x186/0x310 fs/namei.c:3962
 __do_sys_rmdir fs/namei.c:3980 [inline]
 __se_sys_rmdir fs/namei.c:3978 [inline]
 __x64_sys_rmdir+0x2c/0x30 fs/namei.c:3978
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 21013 Comm: syz-executor.1 Tainted: G        W         5.12.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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