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BUG: KCSAN: data-race in ext4_ext_insert_extent / ext4_mark_iloc_dirty

write to 0xffff88810d6e9a36 of 2 bytes by task 391 on cpu 0:
 ext4_ext_store_pblock fs/ext4/ext4_extents.h:256 [inline]
 ext4_ext_insert_extent+0x19e4/0x2b50 fs/ext4/extents.c:2116
 ext4_ext_map_blocks+0x16a3/0x1fc0 fs/ext4/extents.c:4259
 ext4_map_blocks+0x700/0xfc0 fs/ext4/inode.c:637
 mpage_map_one_extent+0x10d/0x390 fs/ext4/inode.c:2396
 mpage_map_and_submit_extent fs/ext4/inode.c:2449 [inline]
 ext4_writepages+0xa51/0x1e50 fs/ext4/inode.c:2801
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x84/0x560 fs/fs-writeback.c:1461
 writeback_sb_inodes+0x6a0/0x1020 fs/fs-writeback.c:1721
 wb_writeback+0x27d/0x660 fs/fs-writeback.c:1894
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2039
 wb_workfn+0xb8/0x410 fs/fs-writeback.c:2080
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

read to 0xffff88810d6e9a34 of 4 bytes by task 24 on cpu 1:
 ext4_do_update_inode fs/ext4/inode.c:5114 [inline]
 ext4_mark_iloc_dirty+0xe0d/0x16e0 fs/ext4/inode.c:5725
 __ext4_mark_inode_dirty+0x4cd/0x5d0 fs/ext4/inode.c:5919
 ext4_convert_unwritten_extents+0x1f6/0x2c0 fs/ext4/extents.c:4768
 ext4_convert_unwritten_io_end_vec+0xa4/0x130 fs/ext4/extents.c:4799
 ext4_end_io_end fs/ext4/page-io.c:184 [inline]
 ext4_do_flush_completed_IO fs/ext4/page-io.c:257 [inline]
 ext4_end_io_rsv_work+0x198/0x360 fs/ext4/page-io.c:271
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x635/0xb90 kernel/workqueue.c:2421
 kthread+0x1fd/0x220 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:296

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 24 Comm: kworker/u4:1 Not tainted 5.11.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: ext4-rsv-conversion ext4_end_io_rsv_work
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