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BUG: KCSAN: data-race in cgroup_freezer_migrate_task / handle_mm_fault

write to 0xffff888029072510 of 2 bytes by task 24390 on cpu 0:
 mem_cgroup_exit_user_fault include/linux/memcontrol.h:864 [inline]
 handle_mm_fault+0x14e4/0x1810 mm/memory.c:4623
 do_user_addr_fault+0x43e/0x830 arch/x86/mm/fault.c:1393
 handle_page_fault arch/x86/mm/fault.c:1450 [inline]
 exc_page_fault+0x94/0x2a0 arch/x86/mm/fault.c:1506
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:580

read to 0xffff888029072510 of 2 bytes by task 1 on cpu 1:
 cgroup_freezer_migrate_task+0xa9/0x290 kernel/cgroup/freezer.c:239
 cgroup_migrate_execute+0x2dc/0x8b0 kernel/cgroup/cgroup.c:2428
 cgroup_migrate kernel/cgroup/cgroup.c:2703 [inline]
 cgroup_attach_task+0x1aa/0x300 kernel/cgroup/cgroup.c:2736
 __cgroup1_procs_write+0x1a7/0x220 kernel/cgroup/cgroup-v1.c:519
 cgroup1_procs_write+0x26/0x30 kernel/cgroup/cgroup-v1.c:532
 cgroup_file_write+0x173/0x2f0 kernel/cgroup/cgroup.c:3696
 kernfs_fop_write_iter+0x1d3/0x2d0 fs/kernfs/file.c:296
 call_write_iter include/linux/fs.h:1901 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x6f0/0x7e0 fs/read_write.c:605
 ksys_write+0xce/0x180 fs/read_write.c:658
 __do_sys_write fs/read_write.c:670 [inline]
 __se_sys_write fs/read_write.c:667 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:667
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 1 Comm: systemd Not tainted 5.11.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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