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BUG: KCSAN: data-race in generic_shutdown_super / kmmpd

write to 0xffff88812d31a050 of 8 bytes by task 1780 on cpu 1:
 generic_shutdown_super+0x61/0x230 fs/super.c:449
 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:1136
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1143
 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:174 [inline]
 exit_to_user_mode_prepare+0x156/0x190 kernel/entry/common.c:208
 __syscall_exit_to_user_mode_work kernel/entry/common.c:290 [inline]
 syscall_exit_to_user_mode+0x20/0x40 kernel/entry/common.c:301
 do_syscall_64+0x56/0x90 arch/x86/entry/common.c:57
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88812d31a050 of 8 bytes by task 21228 on cpu 0:
 sb_rdonly include/linux/fs.h:2221 [inline]
 kmmpd+0x32f/0x6a0 fs/ext4/mmp.c:187
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

value changed: 0x0000000070010000 -> 0x0000000030010000

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 21228 Comm: kmmpd-loop3 Tainted: G        W         5.13.0-rc4-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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