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BUG: KCSAN: data-race in ondemand_readahead / ondemand_readahead

write to 0xffff888103dbb1a4 of 4 bytes by task 18800 on cpu 0:
 ondemand_readahead+0x44e/0x730 mm/readahead.c:472
 page_cache_sync_readahead+0x1b0/0x1e0 mm/readahead.c:527
 generic_file_buffered_read mm/filemap.c:2029 [inline]
 generic_file_read_iter+0xeae/0x1490 mm/filemap.c:2302
 ext4_file_read_iter fs/ext4/file.c:131 [inline]
 ext4_file_read_iter+0x103/0x360 fs/ext4/file.c:114
 call_read_iter include/linux/fs.h:1895 [inline]
 generic_file_splice_read+0x2df/0x470 fs/splice.c:313
 do_splice_to+0xc7/0x100 fs/splice.c:892
 splice_direct_to_actor+0x1b9/0x540 fs/splice.c:971
 do_splice_direct+0x152/0x1d0 fs/splice.c:1080
 do_sendfile+0x396/0x810 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1581 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0x121/0x140 fs/read_write.c:1567
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888103dbb1a4 of 4 bytes by task 18799 on cpu 1:
 ondemand_readahead+0x124/0x730 mm/readahead.c:409
 page_cache_sync_readahead+0x1b0/0x1e0 mm/readahead.c:527
 generic_file_buffered_read mm/filemap.c:2029 [inline]
 generic_file_read_iter+0xeae/0x1490 mm/filemap.c:2302
 ext4_file_read_iter fs/ext4/file.c:131 [inline]
 ext4_file_read_iter+0x103/0x360 fs/ext4/file.c:114
 call_read_iter include/linux/fs.h:1895 [inline]
 generic_file_splice_read+0x2df/0x470 fs/splice.c:313
 do_splice_to+0xc7/0x100 fs/splice.c:892
 splice_direct_to_actor+0x1b9/0x540 fs/splice.c:971
 do_splice_direct+0x152/0x1d0 fs/splice.c:1080
 do_sendfile+0x396/0x810 fs/read_write.c:1520
 __do_sys_sendfile64 fs/read_write.c:1581 [inline]
 __se_sys_sendfile64 fs/read_write.c:1567 [inline]
 __x64_sys_sendfile64+0x121/0x140 fs/read_write.c:1567
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18799 Comm: syz-executor.2 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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