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BUG: KCSAN: data-race in __ext4_journal_start_sb / ext4_remount

write to 0xffff888102c98050 of 8 bytes by task 31938 on cpu 0:
 ext4_remount+0x3ca/0x1840 fs/ext4/super.c:5891
 legacy_reconfigure+0x8e/0xa0 fs/fs_context.c:615
 reconfigure_super+0x2d3/0x4f0 fs/super.c:906
 do_remount fs/namespace.c:2639 [inline]
 path_mount+0x1442/0x1ce0 fs/namespace.c:3227
 do_mount fs/namespace.c:3248 [inline]
 __do_sys_mount fs/namespace.c:3456 [inline]
 __se_sys_mount+0x23d/0x2e0 fs/namespace.c:3433
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3433
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888102c98050 of 8 bytes by task 31931 on cpu 1:
 sb_rdonly include/linux/fs.h:2223 [inline]
 ext4_journal_check_start fs/ext4/ext4_jbd2.c:73 [inline]
 __ext4_journal_start_sb+0x84/0x2d0 fs/ext4/ext4_jbd2.c:98
 __ext4_journal_start fs/ext4/ext4_jbd2.h:320 [inline]
 ext4_da_write_begin+0x410/0x820 fs/ext4/inode.c:2996
 generic_perform_write+0x196/0x3a0 mm/filemap.c:3676
 ext4_buffered_write_iter+0x2f2/0x3f0 fs/ext4/file.c:269
 ext4_file_write_iter+0x2e7/0x11d0 fs/ext4/file.c:519
 call_write_iter include/linux/fs.h:2116 [inline]
 new_sync_write fs/read_write.c:518 [inline]
 vfs_write+0x69d/0x770 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+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 31931 Comm: syz-executor.3 Tainted: G        W         5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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