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BUG: KCSAN: data-race in test_clear_page_writeback / writeout_period

write to 0xffffffff86d9f080 of 8 bytes by interrupt on cpu 1:
 writeout_period+0xca/0xf0 mm/page-writeback.c:635
 call_timer_fn+0x58/0x2e0 kernel/time/timer.c:1405
 expire_timers kernel/time/timer.c:1450 [inline]
 __run_timers kernel/time/timer.c:1774 [inline]
 __run_timers kernel/time/timer.c:1741 [inline]
 run_timer_softirq+0xb84/0xbd0 kernel/time/timer.c:1789
 __do_softirq+0x118/0x34a kernel/softirq.c:292
 invoke_softirq kernel/softirq.c:373 [inline]
 irq_exit+0xb5/0xd0 kernel/softirq.c:413
 exiting_irq arch/x86/include/asm/apic.h:546 [inline]
 smp_apic_timer_interrupt+0xe2/0x270 arch/x86/kernel/apic/apic.c:1146
 apic_timer_interrupt+0xf/0x20 arch/x86/entry/entry_64.S:829
 arch_local_irq_restore arch/x86/include/asm/paravirt.h:752 [inline]
 kcsan_setup_watchpoint+0x2d3/0x400 kernel/kcsan/core.c:513
 compound_head include/linux/page-flags.h:174 [inline]
 PageSwapBacked include/linux/page-flags.h:340 [inline]
 PageSwapCache include/linux/page-flags.h:385 [inline]
 page_mapping+0x12b/0x290 mm/util.c:668
 clear_page_dirty_for_io+0x3f/0x5a0 mm/page-writeback.c:2655
 mpage_submit_page+0x64/0x190 fs/ext4/inode.c:2054
 mpage_map_and_submit_buffers+0x35d/0x610 fs/ext4/inode.c:2318
 mpage_map_and_submit_extent fs/ext4/inode.c:2457 [inline]
 ext4_writepages+0x1176/0x1d40 fs/ext4/inode.c:2772
 do_writepages+0x6b/0x170 mm/page-writeback.c:2344
 __filemap_fdatawrite_range+0x1bb/0x220 mm/filemap.c:421
 file_write_and_wait_range+0xfb/0x160 mm/filemap.c:760
 ext4_sync_file+0x26e/0x8b0 fs/ext4/fsync.c:153
 vfs_fsync_range+0x7c/0x150 fs/sync.c:197
 generic_write_sync include/linux/fs.h:2867 [inline]
 ext4_buffered_write_iter+0x211/0x280 fs/ext4/file.c:277
 ext4_file_write_iter+0xf4/0xd30 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write+0x303/0x400 fs/read_write.c:483
 __vfs_write+0x9e/0xb0 fs/read_write.c:496
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x189/0x380 fs/read_write.c:542
 ksys_write+0xc5/0x1a0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x49/0x60 fs/read_write.c:620
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffffffff86d9f080 of 8 bytes by task 423 on cpu 0:
 wb_domain_writeout_inc mm/page-writeback.c:584 [inline]
 __wb_writeout_inc mm/page-writeback.c:605 [inline]
 test_clear_page_writeback+0x57f/0x7e0 mm/page-writeback.c:2737
 end_page_writeback+0x98/0x180 mm/filemap.c:1317
 ext4_finish_bio+0x430/0x540 fs/ext4/page-io.c:148
 ext4_release_io_end+0xc5/0x270 fs/ext4/page-io.c:163
 ext4_end_io_end fs/ext4/page-io.c:197 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:262 [inline]
 ext4_end_io_rsv_work+0x159/0x400 fs/ext4/page-io.c:276
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 423 Comm: kworker/u4:3 Not tainted 5.6.0-rc1-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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