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BUG: KCSAN: data-race in write_cache_pages / write_cache_pages

write to 0xffff888102607660 of 8 bytes by task 8475 on cpu 1:
 write_cache_pages+0x7ab/0x7f0 mm/page-writeback.c:2297
 generic_writepages+0x64/0xa0 mm/page-writeback.c:2337
 blkdev_writepages+0x19/0x20 fs/block_dev.c:1946
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19d/0x1d0 mm/filemap.c:422
 __filemap_fdatawrite mm/filemap.c:430 [inline]
 filemap_flush+0x1f/0x30 mm/filemap.c:457
 __sync_blockdev+0x69/0x70 fs/block_dev.c:508
 __sync_filesystem fs/sync.c:40 [inline]
 sync_filesystem+0xcb/0x160 fs/sync.c:64
 generic_shutdown_super+0x3c/0x1f0 fs/super.c:448
 kill_block_super+0x4a/0x90 fs/super.c:1446
 deactivate_locked_super+0x6a/0xb0 fs/super.c:335
 deactivate_super+0x85/0x90 fs/super.c:366
 cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:151
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:164 [inline]
 exit_to_user_mode_prepare+0x13c/0x170 kernel/entry/common.c:191
 syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266
 do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

write to 0xffff888102607660 of 8 bytes by task 26 on cpu 0:
 write_cache_pages+0x7ab/0x7f0 mm/page-writeback.c:2297
 generic_writepages+0x64/0xa0 mm/page-writeback.c:2337
 blkdev_writepages+0x19/0x20 fs/block_dev.c:1946
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x560 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6a0/0x1020 fs/fs-writeback.c:1721
 __writeback_inodes_wb+0xb0/0x2a0 fs/fs-writeback.c:1790
 wb_writeback+0x290/0x660 fs/fs-writeback.c:1896
 wb_check_background_flush fs/fs-writeback.c:1964 [inline]
 wb_do_writeback+0x583/0x5d0 fs/fs-writeback.c:2052
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2272
 worker_thread+0x635/0xb90 kernel/workqueue.c:2418
 kthread+0x1fa/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26 Comm: kworker/u4:2 Not tainted 5.10.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-7:1)
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