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BUG: KCSAN: data-race in mm_update_next_owner / start_this_handle

write to 0xffff888103acc02c of 4 bytes by task 8414 on cpu 1:
 memalloc_nofs_save include/linux/sched/mm.h:244 [inline]
 start_this_handle+0xf4c/0x1010 fs/jbd2/transaction.c:452
 jbd2__journal_start+0x1fc/0x3f0 fs/jbd2/transaction.c:503
 __ext4_journal_start_sb+0x154/0x310 fs/ext4/ext4_jbd2.c:105
 __ext4_journal_start fs/ext4/ext4_jbd2.h:320 [inline]
 ext4_dirty_inode+0x78/0xc0 fs/ext4/inode.c:5951
 __mark_inode_dirty+0x72/0x6b0 fs/fs-writeback.c:2262
 generic_update_time fs/inode.c:1763 [inline]
 update_time fs/inode.c:1776 [inline]
 touch_atime+0x1b1/0x2d0 fs/inode.c:1847
 file_accessed include/linux/fs.h:2218 [inline]
 iterate_dir+0x31b/0x370 fs/readdir.c:70
 __do_sys_getdents64 fs/readdir.c:366 [inline]
 __se_sys_getdents64+0x7f/0x190 fs/readdir.c:351
 __x64_sys_getdents64+0x3f/0x50 fs/readdir.c:351
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888103acc02c of 4 bytes by task 15514 on cpu 0:
 mm_update_next_owner+0x1d9/0x4e0 kernel/exit.c:388
 exit_mm+0x337/0x430 kernel/exit.c:500
 do_exit+0x41f/0x1690 kernel/exit.c:812
 do_group_exit+0x17d/0x180 kernel/exit.c:922
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:933
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:931
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:931
 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: 0 PID: 15514 Comm: syz-executor.5 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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