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BUG: KCSAN: data-race in __send_signal / page_cache_readahead_unbounded

write to 0xffff888121b30024 of 4 bytes by task 8693 on cpu 0:
 memalloc_nofs_save include/linux/sched/mm.h:253 [inline]
 page_cache_readahead_unbounded+0xa6/0x470 mm/readahead.c:199
 ondemand_readahead+0x4e4/0x6e0 mm/readahead.c:273
 page_cache_async_readahead+0x1c6/0x1e0 mm/readahead.c:631
 do_async_mmap_readahead mm/filemap.c:2630 [inline]
 filemap_fault+0x2ef/0x1270 mm/filemap.c:2685
 ext4_filemap_fault+0x4b/0x60 fs/ext4/inode.c:6060
 __do_fault mm/memory.c:3462 [inline]
 do_shared_fault+0xc3/0x7c0 mm/memory.c:3909
 do_fault mm/memory.c:3987 [inline]
 handle_pte_fault mm/memory.c:4223 [inline]
 __handle_mm_fault mm/memory.c:4358 [inline]
 handle_mm_fault+0x1710/0x1af0 mm/memory.c:4456
 do_user_addr_fault+0x380/0x770 arch/x86/mm/fault.c:1372
 handle_page_fault arch/x86/mm/fault.c:1429 [inline]
 exc_page_fault+0xb8/0x330 arch/x86/mm/fault.c:1482
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:538

read to 0xffff888121b30024 of 4 bytes by task 8692 on cpu 1:
 __send_signal+0x26d/0x7d0 kernel/signal.c:1098
 send_signal+0x398/0x3e0 kernel/signal.c:1241
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1290
 do_send_specific+0xc7/0x100 kernel/signal.c:3798
 do_tkill kernel/signal.c:3824 [inline]
 __do_sys_tgkill kernel/signal.c:3843 [inline]
 __se_sys_tgkill kernel/signal.c:3837 [inline]
 __x64_sys_tgkill+0x108/0x140 kernel/signal.c:3837
 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: 8692 Comm: syz-fuzzer Not tainted 5.9.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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