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BUG: KCSAN: data-race in __add_to_page_cache_locked / ext4_writepages

write to 0xffff8881140ea7c8 of 8 bytes by task 1754 on cpu 0:
 __add_to_page_cache_locked+0x2cc/0x710 mm/filemap.c:868
 add_to_page_cache_lru+0xa0/0x1b0 mm/filemap.c:919
 generic_file_buffered_read+0xece/0x1fa0 mm/filemap.c:2409
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2507
 ext4_file_read_iter+0x2d8/0x420 fs/ext4/file.c:74
 call_read_iter include/linux/fs.h:1876 [inline]
 generic_file_splice_read+0x22b/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:890 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:970
 do_splice_direct+0xf2/0x170 fs/splice.c:1079
 do_sendfile+0x56a/0xba0 fs/read_write.c:1548
 __do_sys_sendfile64 fs/read_write.c:1609 [inline]
 __se_sys_sendfile64 fs/read_write.c:1595 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1595
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881140ea7c8 of 8 bytes by task 1761 on cpu 1:
 ext4_writepages+0x139/0x1ed0 fs/ext4/inode.c:2649
 do_writepages+0x7b/0x150 mm/page-writeback.c:2352
 __filemap_fdatawrite_range+0x19b/0x1d0 mm/filemap.c:422
 filemap_write_and_wait_range+0x8b/0x2a0 mm/filemap.c:655
 iomap_dio_rw+0x371/0x980 fs/iomap/direct-io.c:478
 ext4_dio_read_iter fs/ext4/file.c:77 [inline]
 ext4_file_read_iter+0x3be/0x420 fs/ext4/file.c:129
 call_read_iter include/linux/fs.h:1876 [inline]
 generic_file_splice_read+0x22b/0x310 fs/splice.c:312
 do_splice_to fs/splice.c:890 [inline]
 splice_direct_to_actor+0x2a8/0x660 fs/splice.c:970
 do_splice_direct+0xf2/0x170 fs/splice.c:1079
 do_sendfile+0x56a/0xba0 fs/read_write.c:1548
 __do_sys_sendfile64 fs/read_write.c:1609 [inline]
 __se_sys_sendfile64 fs/read_write.c:1595 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1595
 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: 1761 Comm: syz-executor.3 Not tainted 5.9.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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