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BUG: KCSAN: data-race in mm_update_next_owner / page_cache_readahead_unbounded

write to 0xffff888102c4a064 of 4 bytes by task 15511 on cpu 0:
 memalloc_nofs_save include/linux/sched/mm.h:253 [inline]
 page_cache_readahead_unbounded+0xa6/0x470 mm/readahead.c:199
 __do_page_cache_readahead mm/readahead.c:273 [inline]
 force_page_cache_readahead+0x1ec/0x220 mm/readahead.c:303
 page_cache_sync_readahead+0x191/0x1a0 mm/readahead.c:580
 generic_file_buffered_read+0x2de/0x1fa0 mm/filemap.c:2199
 generic_file_read_iter+0x7d/0x3e0 mm/filemap.c:2507
 blkdev_read_iter+0xb3/0xc0 fs/block_dev.c:1927
 call_read_iter include/linux/fs.h:1876 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x57a/0x5e0 fs/read_write.c:481
 ksys_read+0xce/0x180 fs/read_write.c:615
 __do_sys_read fs/read_write.c:625 [inline]
 __se_sys_read fs/read_write.c:623 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:623
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff888102c4a064 of 4 bytes by task 17174 on cpu 1:
 mm_update_next_owner+0x1d9/0x4a0 kernel/exit.c:387
 exit_mm+0x2c0/0x330 kernel/exit.c:482
 do_exit+0x40f/0x16c0 kernel/exit.c:793
 do_group_exit+0x17a/0x180 kernel/exit.c:903
 __do_sys_exit_group+0xb/0x10 kernel/exit.c:914
 __se_sys_exit_group+0x5/0x10 kernel/exit.c:912
 __x64_sys_exit_group+0x16/0x20 kernel/exit.c:912
 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: 17174 Comm: syz-executor.0 Not tainted 5.9.0-rc8-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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