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BUG: KCSAN: data-race in page_cache_ra_unbounded / prepare_signal

write to 0xffff88810b4e702c of 4 bytes by task 8359 on cpu 1:
 memalloc_nofs_save include/linux/sched/mm.h:244 [inline]
 page_cache_ra_unbounded+0xae/0x4c0 mm/readahead.c:193
 do_page_cache_ra+0xa7/0xc0 mm/readahead.c:267
 do_sync_mmap_readahead+0x32a/0x360 mm/filemap.c:2810
 filemap_fault+0x1d2/0xc30 mm/filemap.c:2898
 ext4_filemap_fault+0x4b/0x60 fs/ext4/inode.c:6194
 __do_fault mm/memory.c:3635 [inline]
 do_shared_fault+0xc3/0x680 mm/memory.c:3991
 do_fault mm/memory.c:4069 [inline]
 handle_pte_fault mm/memory.c:4316 [inline]
 __handle_mm_fault mm/memory.c:4451 [inline]
 handle_mm_fault+0x12b0/0x1840 mm/memory.c:4549
 do_user_addr_fault+0x60c/0xc00 arch/x86/mm/fault.c:1390
 handle_page_fault arch/x86/mm/fault.c:1475 [inline]
 exc_page_fault+0x94/0x2a0 arch/x86/mm/fault.c:1531
 asm_exc_page_fault+0x1e/0x30 arch/x86/include/asm/idtentry.h:577

read to 0xffff88810b4e702c of 4 bytes by task 8360 on cpu 0:
 sig_task_ignored kernel/signal.c:94 [inline]
 sig_ignored kernel/signal.c:119 [inline]
 prepare_signal+0x872/0xd80 kernel/signal.c:962
 __send_signal+0x63/0x780 kernel/signal.c:1082
 send_signal+0x398/0x3e0 kernel/signal.c:1242
 do_send_sig_info+0x9b/0xe0 kernel/signal.c:1291
 do_send_specific+0xc7/0x100 kernel/signal.c:3819
 do_tkill kernel/signal.c:3845 [inline]
 __do_sys_tgkill kernel/signal.c:3864 [inline]
 __se_sys_tgkill kernel/signal.c:3858 [inline]
 __x64_sys_tgkill+0x108/0x140 kernel/signal.c:3858
 do_syscall_64+0x39/0x80 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 8360 Comm: syz-fuzzer Not tainted 5.12.0-rc2-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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