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BUG: KCSAN: data-race in test_clear_page_writeback / writeout_period

write to 0xffffffff8456f378 of 8 bytes by interrupt on cpu 0:
 writeout_period+0xa6/0xe0 mm/page-writeback.c:623
 call_timer_fn+0x2e/0x1d0 kernel/time/timer.c:1431
 expire_timers+0x135/0x250 kernel/time/timer.c:1476
 __run_timers+0x358/0x420 kernel/time/timer.c:1745
 run_timer_softirq+0x2d/0x30 kernel/time/timer.c:1760
 __do_softirq+0x12c/0x26e kernel/softirq.c:558
 invoke_softirq kernel/softirq.c:432 [inline]
 __irq_exit_rcu+0x9a/0xb0 kernel/softirq.c:636
 sysvec_apic_timer_interrupt+0x32/0x80 arch/x86/kernel/apic/apic.c:1100
 asm_sysvec_apic_timer_interrupt+0x12/0x20 arch/x86/include/asm/idtentry.h:638

read to 0xffffffff8456f378 of 8 bytes by task 1848 on cpu 1:
 wb_domain_writeout_inc mm/page-writeback.c:572 [inline]
 __wb_writeout_inc mm/page-writeback.c:593 [inline]
 test_clear_page_writeback+0x387/0x560 mm/page-writeback.c:2754
 end_page_writeback+0xd7/0x1b0 mm/filemap.c:1548
 ext4_finish_bio+0x3e2/0x430 fs/ext4/page-io.c:143
 ext4_release_io_end+0x98/0x200 fs/ext4/page-io.c:158
 ext4_end_io_end fs/ext4/page-io.c:192 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:257 [inline]
 ext4_end_io_rsv_work+0x306/0x360 fs/ext4/page-io.c:271
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

value changed: 0x00000000ffffc16c -> 0x00000000ffffc298

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1848 Comm: kworker/u4:7 Not tainted 5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
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