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BUG: KCSAN: data-race in write_cache_pages / write_cache_pages

write to 0xffff88810108dc30 of 8 bytes by task 505 on cpu 0:
 write_cache_pages+0x7f7/0x840 mm/page-writeback.c:2306
 generic_writepages+0x7c/0xc0 mm/page-writeback.c:2346
 blkdev_writepages+0x19/0x20 block/fops.c:374
 do_writepages+0x26b/0x3c0 mm/page-writeback.c:2364
 filemap_fdatawrite_wbc mm/filemap.c:400 [inline]
 __filemap_fdatawrite_range mm/filemap.c:433 [inline]
 __filemap_fdatawrite mm/filemap.c:439 [inline]
 filemap_flush+0x154/0x180 mm/filemap.c:466
 __sync_blockdev+0x69/0x70 block/bdev.c:192
 __sync_filesystem fs/sync.c:40 [inline]
 sync_filesystem+0xcb/0x160 fs/sync.c:64
 generic_shutdown_super+0x42/0x230 fs/super.c:448
 kill_block_super+0x4a/0x90 fs/super.c:1395
 deactivate_locked_super+0x65/0xd0 fs/super.c:335
 deactivate_super+0x85/0x90 fs/super.c:366
 cleanup_mnt+0x267/0x2d0 fs/namespace.c:1137
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1144
 task_work_run+0xae/0x130 kernel/task_work.c:164
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
 exit_to_user_mode_prepare+0x156/0x190 kernel/entry/common.c:207
 __syscall_exit_to_user_mode_work kernel/entry/common.c:289 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:300
 do_syscall_64+0x50/0xa0 arch/x86/entry/common.c:86
 entry_SYSCALL_64_after_hwframe+0x44/0xae

write to 0xffff88810108dc30 of 8 bytes by task 511 on cpu 1:
 write_cache_pages+0x7f7/0x840 mm/page-writeback.c:2306
 generic_writepages+0x7c/0xc0 mm/page-writeback.c:2346
 blkdev_writepages+0x19/0x20 block/fops.c:374
 do_writepages+0x26b/0x3c0 mm/page-writeback.c:2364
 __writeback_single_inode+0x80/0x440 fs/fs-writeback.c:1616
 writeback_sb_inodes+0x563/0xd30 fs/fs-writeback.c:1881
 __writeback_inodes_wb fs/fs-writeback.c:1950 [inline]
 wb_writeback+0x31f/0x6d0 fs/fs-writeback.c:2055
 wb_check_background_flush fs/fs-writeback.c:2121 [inline]
 wb_do_writeback+0x4e4/0x530 fs/fs-writeback.c:2209
 wb_workfn+0xc8/0x540 fs/fs-writeback.c:2237
 process_one_work+0x402/0x910 kernel/workqueue.c:2297
 worker_thread+0x636/0xae0 kernel/workqueue.c:2444
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30

value changed: 0x0000000000000000 -> 0x0000000000000004

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 511 Comm: kworker/u4:3 Not tainted 5.15.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-7:3)
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