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BUG: KCSAN: data-race in ext4_write_end / ext4_writepages

write to 0xffff8880096107f0 of 8 bytes by task 3503 on cpu 0:
 i_size_write include/linux/fs.h:872 [inline]
 ext4_update_inode_size fs/ext4/ext4.h:3289 [inline]
 ext4_write_end+0x1fe/0xa00 fs/ext4/inode.c:1311
 ext4_da_write_end+0x93/0x760 fs/ext4/inode.c:3081
 generic_perform_write+0x23e/0x3a0 mm/filemap.c:3586
 ext4_buffered_write_iter+0x2e5/0x3e0 fs/ext4/file.c:269
 ext4_file_write_iter+0x48a/0x10b0 fs/ext4/file.c:502
 call_write_iter include/linux/fs.h:1977 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x7a6/0xe20 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1325 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1311
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8880096107f0 of 8 bytes by task 28350 on cpu 1:
 i_size_read include/linux/fs.h:850 [inline]
 mpage_submit_page fs/ext4/inode.c:2096 [inline]
 mpage_map_and_submit_buffers fs/ext4/inode.c:2350 [inline]
 mpage_map_and_submit_extent fs/ext4/inode.c:2489 [inline]
 ext4_writepages+0xe21/0x1e70 fs/ext4/inode.c:2802
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __writeback_single_inode+0x80/0x4d0 fs/fs-writeback.c:1467
 writeback_sb_inodes+0x6a7/0xff0 fs/fs-writeback.c:1732
 wb_writeback+0x27d/0x660 fs/fs-writeback.c:1905
 wb_do_writeback+0x101/0x5d0 fs/fs-writeback.c:2050
 wb_workfn+0xc8/0x430 fs/fs-writeback.c:2091
 process_one_work+0x3e1/0x950 kernel/workqueue.c:2275
 worker_thread+0x616/0xa70 kernel/workqueue.c:2421
 kthread+0x20b/0x230 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 28350 Comm: kworker/u4:1 Tainted: G        W         5.12.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: writeback wb_workfn (flush-8:0)
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